Watford missed out on automatic promotion after losing to Leeds in a game delayed by serious injury to keeper Jonathan Bond.
Dominic Poleon put Leeds in front, but Almen Abdi levelled shortly after.
Watford's Troy Deeney then saw red for two bookable offences before news of a
late Cardiff equaliser at Hull
left the Hornets needing a goal to go up.
But Ross McCormack scored Leeds' winner with the help of a blunder by replacement keeper Jack Bonham.
Watford's play-off dates
Thursday 9 May:
Leicester City v Watford (19:45 BST)
Sunday 12 May:
Watford v Leicester City (12:30 BST)
Watford will now turn to a play-off semi-final against Leicester City in pursuit of promotion and will hope to recover from an emotionally draining day to replicate their play-off victories in 1999 and 2006.
The Hornets have been out of the top flight for six years now, but have been genuine promotion contenders all season with the dawning of a new era under
owners the Pozzo family.
The Italians have injected life into the Hertfordshire club, firstly by appointing Gianfranco Zola as boss and then by bringing in players on loan from Udinese and Granada - both clubs also owned by the family.
It has been a controversial approach, with nine members of the current squad belonging to either the Italian or Spanish sides, but it has unquestionably been effective.
Both Watford and Leeds were unsettled by late changes before kick-off, with Hornets keeper Manuel Almunia replaced by Bond and United midfielder Zac Thompson making way for Michael Brown.
And the changes continued early on, with Leeds' Steve Morison taking a blow to the head and being replaced by Poleon, who was at close quarters when Bond collided with his own defender and was taken off on a stretcher to make way for the entrance of 19-year-old academy product Bonham.
It was Poleon who would then get the goal to put his side in front, rolling into an empty net after Bonham and Joel Ekstrand got into a muddle trying to deal with Tom Lees's long ball.
Watford form heading into the play-offs
The Hornets end the Championship season having won only two of their final six league games
Watford's deflation was short lived however when Abdi collected the ball from Ikechi Anya, who had surged in off the touchline, and curled a delightful finish beyond Paddy Kenny from 20 yards out.
Brian McDermott, who was winning promotion with Reading this time last year, will be pleased his mid-table Leeds side arrived with the commitment and desire to play their part in the final day permutations.
His men became withdrawn late on though, particularly after Deeney's second yellow card for a challenge on Michael Brown.
With the game heavily delayed by the earlier injuries, news of Cardiff's equaliser at Hull filtered through the crowd and the Hornets, urged on by a wave of noise, hunted the goal that would put them into the Premier League, while the Tigers could only sit back and watch.
Kenny made a fine double save from Abdi's shot and Nathaniel Chalobah's follow-up to kept the home side at bay.
And as Zola's men threw everything forward, Leeds finished off their promotion dreams when Aiden White broke clear and fed McCormack, who delivered a lob that debutant Bonham failed to stop, despite getting a good hand to it.
McDermott will look to mould his own promotion charge next season off the back of this final day flourish, while for Watford that ambition is still very much a possibility this term, but they will have to do it the hard way.
Watford boss Gianfranco Zola:
"I was shouting to my players, encouraging them, but they had given everything. I cannot complain.
"Manuel Almunia had a problem in his left hamstring. That's why we had to go with Jonathan Bond.
"With Jonathan the first impression is he has a broken nose. But he's been taken to hospital for assessment.
"It looks like it's not too bad for Almunia and we're hopeful he can be back [to face Leicester in Thursday] as we know Leicester will be a tough game."
Leeds boss Brian McDermott:
"It was an amazing game. We kind of knew what was going on at Hull, but we did what we had to do.
"We were spot on and did our business for our club. Hopefully Jonathan Bond is OK. Steve Morison has concussion.
"Dominic Poleon is a young boy from the academy with real pace and power. We need to get him really fit, if we do that this summer he could be a real asset to us.
"Gianfranco is a friend of mine. I hope Watford do well as Gianfranco is a top person, he's done a fantastic job and they have a chance in the play-offs."
Full Time The referee blows for the end of the match.
Unfair challenge on Joel Ekstrand by Aidan White results in a free kick. Mark Yeates delivers the ball from the free kick right-footed from right channel, blocked by Jason Pearce.
Inswinging corner taken from the left by-line by Almen Abdi. Inswinging corner taken by Almen Abdi, Lee Peltier manages to make a clearance.
The ball is crossed by Joel Ekstrand.
Mark Yeates fouled by Lee Peltier, the ref awards a free kick. Direct free kick taken by Mark Yeates.
The ball is crossed by Daniel Pudil.
90:00 +0:52 Substitution
Substitution Ryan Hall is brought on as a substitute for Paul Green.
89:25 Goal scored
Goal! - Ross McCormack - Watford 1 - 2 Leeds Goal scored by Ross McCormack from deep inside the penalty box low into the middle of the goal. Watford 1-2 Leeds.
Fernando Forestieri has an effort at goal from deep inside the area missing to the right of the goal. Inswinging corner taken right-footed by Fernando Forestieri, clearance made by Ross McCormack.
Fernando Forestieri is ruled offside. Patrick Kenny restarts play with the free kick.
Ross McCormack is penalised for handball and concedes a free kick. Daniel Pudil restarts play with the free kick.
Booking Daniel Pudil is shown a yellow card for dissent.
Corner from left by-line taken by Almen Abdi.
Nathaniel Chalobah takes a shot.
Aidan White produces a left-footed shot from the edge of the box and misses to the left of the target.
Booking The referee shows Joel Ekstrand a yellow card.
The ball is swung over by Adam Drury, Jason Pearce challenges Daniel Pudil unfairly and gives away a free kick. Direct free kick taken by Jack Bonham.
Substitution Mark Yeates replaces Jonathan Hogg.
Unfair challenge on Ross McCormack by Joel Ekstrand results in a free kick.
Centre by Daniel Pudil, Adam Drury manages to make a clearance.
Unfair challenge on Ross McCormack by Nathaniel Chalobah results in a free kick. Ross McCormack crosses the ball from the free kick right-footed from left channel, Jack Bonham manages to make a clearance.
Aidan White is flagged offside by the assistant referee. Direct free kick taken by Jack Bonham.
The official flags Ross McCormack offside. Jack Bonham takes the free kick.
Inswinging corner taken from the left by-line by Michael Tonge.
Free kick awarded for a foul by Nathaniel Chalobah on Paul Green. Direct free kick taken by Lee Peltier.
Dominic Poleon produces a right-footed shot from just outside the penalty area that goes harmlessly over the target.
Centre by Ross McCormack, Joel Ekstrand manages to make a clearance.
Free kick awarded for a foul by Marco Cassetti on Ross McCormack. Michael Tonge takes the free kick.
Daniel Pudil takes a shot. Michael Tonge gets a block in. Matej Vydra takes a shot. Patrick Kenny makes a fantastic save.
Free kick awarded for an unfair challenge on Almen Abdi by Aidan White. The free kick is swung in right-footed by Almen Abdi, Luke Varney gets a block in.
The ball is delivered by Daniel Pudil.
Almen Abdi has shot on goal from just outside the penalty box which goes wide of the right-hand upright.
Unfair challenge on Nathaniel Chalobah by Michael Tonge results in a free kick. Nathaniel Chalobah takes the free kick.
The ball is swung over by Aidan White, Headed effort from inside the penalty area by Luke Varney misses to the right of the target.
Unfair challenge on Fernando Forestieri by Paul Green results in a free kick. The ball is sent over by Almen Abdi, Close range header by Joel Ekstrand goes wide of the left-hand post.
Almen Abdi produces a cross, clearance made by Patrick Kenny. Save made by Patrick Kenny.
The referee blows for offside against Luke Varney. Jack Bonham takes the free kick.
Substitution Matej Vydra on for Ikechi Anya.
Substitution Aidan White on for Michael Brown.
Paul Green is penalised for handball and concedes a free kick. Jack Bonham takes the direct free kick.
Ross McCormack crosses the ball in from the free kick, Daniel Pudil manages to make a clearance.
Dismissal Troy Deeney is sent off by the referee for second bookable offence.
Troy Deeney sends in a cross, free kick awarded for an unfair challenge on Michael Brown by Troy Deeney.
Paul Green takes a shot. Marco Cassetti gets a block in. Inswinging corner taken by Adam Drury, clearance made by Ikechi Anya.
Almen Abdi takes a shot. Blocked by Jason Pearce.
Foul by Troy Deeney on Paul Green, free kick awarded. Lee Peltier takes the free kick.
Unfair challenge on Lee Peltier by Daniel Pudil results in a free kick. The free kick is swung in left-footed by Adam Drury.
The ball is delivered by Michael Tonge, save by Jack Bonham.
Fernando Forestieri is ruled offside. Direct free kick taken by Patrick Kenny.
The offside flag is raised against Luke Varney. Free kick taken by Jack Bonham.
The ball is sent over by Tom Lees.
Effort from the edge of the area by Ross McCormack goes wide of the right-hand upright.
Nathaniel Chalobah is penalised for handball and concedes a free kick. Direct free kick taken by Patrick Kenny.
Shot by Fernando Forestieri from deep inside the penalty area misses to the left of the target.
The referee blows for offside against Luke Varney. Direct free kick taken by Lloyd Doyley.
The referee blows for offside against Luke Varney. Marco Cassetti restarts play with the free kick.
Jason Pearce challenges Troy Deeney unfairly and gives away a free kick. Jonathan Hogg takes the direct free kick. Handball decision goes against Michael Tonge. Nathaniel Chalobah takes the free kick.
The referee gets the second half started.
45:00 +17:31 Half time
Half Time The whistle is blown to end the first half.
45:00 +15:27 Booking
Booking Caution for Troy Deeney.
Paul Green fouled by Almen Abdi, the ref awards a free kick. Direct free kick taken by Ross McCormack.
Free kick awarded for an unfair challenge on Marco Cassetti by Luke Varney. Marco Cassetti takes the free kick.
Adam Drury delivers the ball, Foul by Michael Brown on Daniel Pudil, free kick awarded. Free kick taken by Jack Bonham.
The ball is swung over by Luke Varney, Marco Cassetti makes a clearance.
Ikechi Anya takes a shot. Save by Patrick Kenny.
Troy Deeney fouled by Tom Lees, the ref awards a free kick. The ball is swung over by Marco Cassetti, Close range headed effort by Almen Abdi misses to the right of the target.
Assist on the goal came from Ikechi Anya.
45:00 +5:30 Goal scored
Goal! - Almen Abdi - Watford 1 - 1 Leeds Almen Abdi finds the back of the net with a goal from just outside the area low into the middle of the goal. Watford 1-1 Leeds.
The ball is delivered by Nathaniel Chalobah, comfortable save by Patrick Kenny.
Nathaniel Chalobah takes a shot. Save made by Patrick Kenny.
41:15 Goal scored
Goal! - Dominic Poleon - Watford 0 - 1 Leeds Dominic Poleon grabs a goal from inside the six-yard box to the bottom right corner of the goal. Watford 0-1 Leeds.
Almen Abdi delivers the ball from the free kick right-footed from right channel, Tom Lees manages to make a clearance.
Booking Michael Brown is booked.
Michael Brown challenges Nathaniel Chalobah unfairly and gives away a free kick.
Jonathan Hogg takes a shot. Blocked by Jason Pearce.
Substitution Jonathan Bond goes off and Jack Bonham comes on.
Booking Dominic Poleon goes into the book for unsporting behaviour.
Adam Drury crosses the ball, clearance by Daniel Pudil.
Free kick awarded for an unfair challenge on Jason Pearce by Troy Deeney. Direct free kick taken by Patrick Kenny.
Almen Abdi takes a shot. Blocked by Jason Pearce.
A cross is delivered by Ikechi Anya, Lee Peltier manages to make a clearance.
The ball is swung over by Ikechi Anya, clearance made by Jason Pearce.
Luke Varney is ruled offside. Lloyd Doyley restarts play with the free kick.
Substitution Dominic Poleon joins the action as a substitute, replacing Steve Morison.
Free kick awarded for an unfair challenge on Daniel Pudil by Lee Peltier. Direct free kick taken by Marco Cassetti.
The official flags Luke Varney offside. Free kick taken by Joel Ekstrand.
Ikechi Anya delivers the ball, Tom Lees makes a clearance.
Effort on goal by Daniel Pudil from 12 yards. Tom Lees gets a block in.
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