Peter Whittingham's solo strike kept Cardiff City in the hunt for automatic promotion and sent Preston down to League One.
Craig Bellamy broke the offside trap early on and picked out Whittingham, who fired his 11th goal of the season into the roof of the net.
Dekel Keinan, Seyi Olofinjana and Chris Burke all had chances for the visitors.
Preston came close when Chris Brown hit the post, and Nathan Ellington had a shot deflected off the line.
It means Phil Brown's side will be playing League One football next season after Crystal Palace beat Leeds United 1-0.
Sheffield United's shock win at Reading piled on more misery as Preston slipped to the bottom of the Championship table.
They put in a spirited display but they could not break down a solid Cardiff side that held firm as they continued their automatic promotion charge, but stay in third after Norwich's last-gasp 3-2 win against Derby County.
It took just six minutes for the Bluebirds to break the deadlock, as Bellamy raced into the box before squaring to the onrushing Whittingham who fired into the roof of the net.
Bellamy almost replicated that move soon after before the Wales striker saw a shot of his own deflected off Leon Cort for a corner.
And from Burke's resulting delivery, defender Keinan sent a free header over the bar from the edge of the box.
Preston showed fight but lacked any real threat up front, leaving visiting goalkeeper Tom Heaton untroubled by a handful of crosses.
As the hosts tried to press, Cardiff were ready to hit them on the break.
Jay Bothroyd had a penalty appeal waved away after he skipped into the box before appearing to go down under a challenge from Preston keeper Iain Turner.
Moments later, another Burke corner was not dealt with and this time Olofinjana volleyed unmarked, only to see Turner tip the ball over.
But Preston weathered the storm and - despite losing top score Iain Hume to injury - they finished the half strongly, seeing four chances go begging in quick succession.
First Ian Ashbee drilled an effort just wide from the edge of the box and then Darren Carter met substitute Darel Russell's cross with a glancing header, only to be denied by the fingertips of Heaton.
Then, from the corner, Brown sent a header against the post and the rebound fell to Ellington whose shot was blocked on the line by his team-mate, Billy Jones.
Cardiff were forced into a defensive reshuffle just after the restart when Kevin McNaughton hobbled off to be replaced by Mark Hudson.
But it was defensive partner Paul Quinn who was almost caught out when Keith Treacy beat him to the ball but the legs of Heaton kept Preston at bay.
Preston were pushing but were caught on the break and Cardiff wasted a chance to put the game to bed.
Bothroyd worked hard to find Burke on the right and his cross was met by Olofinjana, who headed over from 12 yards, before Bellamy smashed an effort goalbound, which Turner palmed away.
Preston felt they should have had a penalty just before the hour mark when Treacy went down under a challenge from Quinn, but loud appeals were turned away by referee Phil Gibbs.
Cardiff soon found their stride, though, and Olofinjana headed another effort wide, this time from Jlloyd Samuel's cross.
Then Burke almost buried Preston in superb fashion, charging through a host of players before a quick one-two with Olofinjana, only to be denied by Turner.
Preston pushed hard once again for a way back in but it was too little, too late and their first relegation in 18 years was confirmed.
Cardiff manager Dave Jones: "We've got to keep applying the pressure. If it's automatic or via the play-offs, we've just got to keep going.
"Playing two games in three games is very difficult but in the first half today we could have been three or four goals up.
"I played here (in the 1980s) but this business is ruthless and I came here with the intention of winning the match, regardless of whatever the outcome was."
Preston boss Phil Brown: "You can say you came in too late or that the previous guy in charge was there too long, but I wouldn't have taken the job if I didn't think I could keep the club in the Championship.
"So I hold myself responsible for the relegation. If you look back, we didn't get our first win until too late and after that it was catch-up time. The luck that evaded us has evaded us again today."