Saints suffered more Heineken Cup heartache at the hands of Leinster as the Irish giants surged to a brutal 40-7 bonus-point win at Franklin’s Gardens.
Matt O’Connor’s men ran riot during the first half, scoring four tries before the break through Luke Fitzgerald, with two, Jamie Heaslip and Eoin Reddan.
Inspirational Ireland centre Brian O’Driscoll added a fifth score on 65 minutes to rub further salt in the hugely painful wound, with Fitzgerald going on to complete his hat-trick five minutes later.
Saints’ only consolation came in the form of Lee Dickson’s 68th-minute score and Stephen Myler’s conversion.
Ian Madigan added 10 points with the boot for the away side as the Gardens faithful were left shell-shocked by a club they had seen rip European glory from Saints hands in the 2011 showpiece.
Jim Mallinder’s men barely troubled the Leinster 22 for the majority of the game as basic errors cost them any momentum and any chance of claiming a much-needed victory.
They now head to Dublin next Saturday, knowing only a big win will do as they aim to stay alive in Pool 1, which Leinster now lead from Castres, with Saints eight points adrift of top spot.
The home side were struck by what proved to be a telling blow before kick-off as James Wilson, who had been a doubt for the game, was forced out, with Ken Pisi being moved to full-back and Jamie Elliott coming in on the wing.
Salesi Ma’afu was also ruled out, meaning Tom Mercey came in at tighthead, while Leinster also had to alter their announced team, with Jack McGrath replacing loosehead Cian Healy.
But it was the shifting of Pisi to an unaccustomed full-back role that had the biggest consequences as his first contribution was to knock on when trying to restart the game.
That gave Leinster a scrum and they made the most of the possession, cutting Saints to ribbons to score.
Fitzgerald was the man on the mark, picking up and dotting down after a superb grubber kick from O’Driscoll.
Madigan made the conversion and the Leinster fans were jubilant.
And they were standing to applaud again soon after as a slick move from right to left, which included a stunning pass from O’Driscoll, culminated in another Fitzgerald try.
Madigan missed, but Leinster had flown into a 12-0 lead inside just eight minutes and Saints were looking sea sick.
They were doing there best to add aggression to the game and find a spark, but they were failing to keep the ball in hand.
Madigan slashed a wild penalty past the left upright but Saints weren’t let off the hook for long as some more stunning rugby saw Rob Kearney send skipper Heaslip over.
Madigan made it 19-0 after 28 minutes and Leinster were already sensing the bonus point, with Saints just desperate for a foothold in the game.
But they weren’t allowed to get it, in any department, as Leinster pounced again before half-time.
Scrum-half Reddan saw another gap in the Saints rearguard and darted through it to score, with Madigan making it 26-0 before the break.
The Gardens was left dumfounded as the shrill sound of the referee’s whistle filled the air, and it looked like things had got even worse as Fitzgerald flew over again.
But this time Saints were saved by the TMO, who deemed hooker Sean Cronin’s final pass had gone forward.
If Northampton had been given no reprieves during the first half, they had two in the space of 15 second-half minutes as Reddan knocked on with the line in sight after another intelligent offload from O’Driscoll.
But they weren’t to get away with it for much longer as the majestic O’Driscoll strolled in for an interception try.
The centre sat in the in goal area struggling to believe how comprehensive it had been for his team before Madigan made it 33-0 with the conversion.
Saints did avoid the ignominy of failing to score a point as Dickson finished off a patient move, with Leinster replacement prop Michael Bent seeing yellow.
It was to make no difference, though, as Fitzgerald completed his hat-trick 10 minutes before the end to leave some home fans heading for the exits.
The ones who remained applauded O’Driscoll on his way off the pitch before the referee brought an end to the horror show soon after.
Saints: K Pisi; Elliott (Collins 72), Waldouck, Burrell, North; Myler (G Dickson 68), L Dickson (Glynn 68); A Waller (E Waller 58), Hartley (c) (Haywood 58), Mercey (Denman 60), Manoa, Lawes, Wood, Dowson (Clark 58), Dickinson (C Day 40).
Leinster: R Kearney, D Kearney (Kirchner 54), O’Driscoll (Gopperth 71), D’Arcy, Fitzgerald; Madigan, Reddan (Cooney 64); McGrath (Bent 64), Cronin (Dundon 68), Ross (Moore 68), Toner, McCarthy (Cullen 60), Ruddock, O’Brien (Jennings 32), Heaslip (c).
Referee: Nigel Owens