A last-gasp Marc Andreu try helped rob Saints of a bonus point as Racing Metro claimed a 20-11 win in the Champions Cup opener in Colombes.
Jim Mallinder’s men had put themselves in contention for victory with five minutes to go as replacement Phil Dowson finally broke the French resistance.
But Andreu popped up after Ben Foden’s kick was charged down to plunge a dagger into Northampton hearts.
Johannes Goosen twisted the knife with the conversion that took Saints out of losing bonus point range as they started life in the new competition slowly.
The Aviva Premiership table-toppers will rue the failure to take anything from a game in which they had their chances and were twice a man up.
Julien Brugnaut and Brian Mujati were both shown yellow cards with the former Saints prop’s departure late on seemingly signalling a late victory charge.
But after Dowson had given the away side a sight of at least a draw, Andreu and Goosen combined to ensure it would not be a happy ending for Saints.
For Racing it was another strong start in Europe as they showed they were up for the fight from the off.
The Top 14 side only won one of their games in the Heineken Cup last season, however it did come in the opening match, at home to Clermont Auvergne.
And they looked desperate to make a strong start in the inaugural Rugby Champions Cup, at the expense of Saints.
Roared on by a vociferous home faithful, backed by an incessant drum beat, Racing opened the scoring, setting up a drive that saw centre Alexandre Dumoulin dot down.
Irish fly-half Jonny Sexton, the Northampton nemesis in the 2011 Heineken Cup final, slotted the conversion and Saints were up against it early on, with the home side suffocating them.
Racing were revelling in the noise being created by their small but passionate support and the home side continued to pose the most danger.
Foden’s kick ahead was seized upon, putting Saints on the back foot, but they managed to scramble in time to stop Andreu getting over the line.
The away side and their sizeable travelling faithful needed a lift, but Racing were defending with real desire, forcing errors from their English opponents.
Saints were also having a tough time in the scrum, giving away penalties that served to relieve any pressure they were able to build up.
But when they did finally find favour with referee George Clancy, they were able to mount a charge that gave Stephen Myler the chance to register from the tee. He took it.
With Foden injecting a spark from full-back, Saints were starting to find their feet, but Racing were attacking and defending with fervour and continued to threaten.
Again, the home side won a penalty at the scrum and Sexton hit the target, sparking another mass flag-waving exercise among the home support.
Saints knew they needed to step things up at the start of the second half and some big carrying from Courtney Lawes brought an early penalty, which Myler landed.
Racing missed the chance to issue an immediate riposte as Sexton’s penalty, which came from Salesi Ma’afu being penalised for an incident with Juandre Kruger, flew wide.
Things were starting to go Saints’ way as Racing loosehead prop Brugnaut was sent to the bin as the away side cranked up the heat at the scrum.
And it looked like Saints would go in front as George Pisi broke free, but, as if hit by a sniper, the Samoan went to ground, dropping the ball just a few metres from the line.
Pisi was forced off with a hamstring injury the cause of his mishap, handing Racing the double boost of remaining unscathed and seeing a creative influence carried off.
Saints’ disappointment was compounded by a Sexton penalty at the other end, with the French side, who had brought Mujati into the fray, seven points up once more.
But Saints would not go down without a fight, applying the pressure after a series of penalties brought a stream of lineout drives.
The travelling support were upping the decibel levels, imploring their team to take the chances they were being given in the home half.
And when Mujati was sent to the bin, Saints seized their opportunity, with Dowson popping up on the right wing to score his side’s first try of the Champions Cup campaign.
With five minutes to go, Myler’s conversion could level the game, but the usually reliable fly-half missed out the chance to square the match.
And Racing rubbed salt in the wounds as Andreu grabbed the ball and dotted down, with Goosen’s kick ensuring Saints would leave Colombes with nothing to show for their efforts.
Racing Metro: Dulin; Imhoff, Dumoulin (Audrin 68), Roberts, Andreu; Sexton (Goosen 74), Machenaud (Phillips 61); Brugnaut, Lacombe (Maurouard 60), Ducalcon (Mujati 61); Kruger, Van der Merwe (Charteris 43); Lauret, La Roux, Claassen.
Saints: Foden; K Pisi, G Pisi (Elliott 54), Burrell, North; Myler, Fotuali’i (L Dickson 54); A Waller (E Waller 63), Hartley (c) (Haywood 63), Ma’afu (Denman 60); Lawes (Dowson 69), C Day (Craig 69); Clark, Wood, Manoa.
Referee: George Clancy (Ireland)