Northampton produced a brilliant display to shock Saracens and reach the Premiership final.
Brian Mujati and Jamie Elliott scored first-half tries as Saints romped to a 17-0 lead at the break.
Sarries, who topped the table and had never lost at Allianz Park, threatened a comeback with two Owen Farrell penalties and Duncan Taylor's score.
Table toppers' semi-final woe
Since the Premiership play-offs were introduced in 2002-03 Saracens are only the second side to have topped the regular season table and then failed to reach the Premiership final - Gloucester being the other in 2008.
But Gerritt-Jan van Velze touched down and Stephen Myler added a penalty to secure the win for Northampton.
Saracens, who had finished top of the Premiership for the first time in their history and were searching for a second title in three years, came into the semi-final as strong favourites.
But Northampton, looking to secure a place in the end-of-season Twickenham showpiece for the first time, started the tie much the brighter.
Myler, who struck the bar with an injury-time kick to hand Saracens a narrow
17-16 victory in December,
twice missed early penalties after problems with his kicking tee.
The 28-year-old, his confidence seemingly dented by the failures, turned down two further opportunities within range as the visitors continued to press.
Their reward duly came, however, when Mujati touched down through a mound of bodies to give Saints a surprise lead, with the video referee confirming the ball had been grounded. Myler successfully converted probably his most difficult chance so far.
Saracens, who had the Premiership's stingiest defence heading into the play-offs, were cut open once more just two minutes later.
Christian Day off-loaded to Elliott, and the winger showed a turn of pace to score. Myler again added the extras.
Myler added another penalty to extend Northampton's advantage further, before Farrell dragged a kick wide of the posts in the dying stages of the opening period to leave the league's table-toppers 17 points in arrears at the interval.
Saracens, knowing a fast start to the second half would be crucial, had a great chance to score within moments of the restart but Ben Foden did brilliantly to prevent David Strettle from touching down.
Saints end top-four hoodoo
Heading into their semi-final clash with Saracens, Northampton had lost their last nine meetings with the other three members of the Premiership's top four.
The hosts eventually got themselves on the scoreboard through Farrell's penalty with 50 minutes on the clock, before the 21-year-old England man added another three points to reduce the deficit to 6-17.
Northampton responded when Lee Dickson teed up replacement Van Velze to burst through and touch down in the corner, with Myler's angled conversion finding the posts to give Saints breathing space once more.
Elliott was then sin-binned for a poor challenge on Farrell and Sarries quickly made their numerical advantage count, switching the ball from right to left to allow Taylor to score. Farrell made no mistake with the conversion.
But it was too little, too late and Myler's penalty with seven minutes left sealed the surprise victory for Saints and set up a Twickenham final against Leicester,
who beat Harlequins on Saturday.
VIEWS FROM THE DRESSING ROOM
Saracens directory of rugby Mark McCall said:
"It is a massive disappointment. We gave ourselves a mountain to climb at half-time. Northampton put us under a lot of pressure, especially in that first half, and we made a lot mistakes.
"Northampton were very, very good today, extremely physical and their defence was outstanding.
"We showed in patches in the second half what we were capable of and we gave it a real go but the first half was very un-Saracens like."
Northampton Saints director of rugby Jim Mallinder told BBC Radio Northampton:
"I'm delighted. It's not been an easy year. A number of people wrote us off and said we couldn't win the big games.
"The performance, intensity and physicality was very, very impressive. In the second half, we hung in there.
"Clearly Leicester will go in as favourites. But it's great to get to a Twickenham final. I can't wait."
Wyles for Tomkins (60), Taylor for Hodgson (25), Wigglesworth for de Kock (49), Gill for Vunipola (66), Smit for Brits (60), Nieto for Stevens (66), Botha for Hargreaves (71), Kruis for Saull (49).
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