Northampton ended the Scarlets' Heineken Cup quarter-final hopes on Saturday with a 29-17 victory at Parc y Scarlets.
The result means that if Munster win against Castres later in the day, they win be guaranteed of top spot in Pool One.
Scarlets led 14-6 at half time with Wales fly-half Rhys Priestland at the origin of all the hosts' points via three penalties and clever cross-field kick to create a try for winger Viliame Iongi.
But the momentum swung towards the Saints in the second period with a Soane Tonga'uiha touchdown and a Ben Foden intercept try in the dying moments sealing victory.
Referee Alain Rolland went some way towards repairing his relationship with Welsh fans early on when he sinbinned Tom May in just the second minute for tamely tossing the ball into the face of Gareth Davies after the whistle had blown.
But the home side managed just a solitary Priestland penalty during their ten minutes with an extra man.
Shortly after Priestland and Stephen Myler exchanged penalties, Rolland reversed a penalty in the Scarlets' favour for another dubious after-the-whistle offence from Calum Clark. Priestland took full advantage to make the score 9-3.
After half-an-hour of petulance and refereeing controversies, a game finally broke out when Priestland executed a cross-field kick perfectly and it was gathered comfortably by Iongi, who put the ball down untouched for the first try.
Myler did add another three points for the Saints before the break but the Llanelli outfit were winning the battle of the breakdown and were good value for their 14-6 half-time lead.
Northampton were grateful for the opportunity to regroup and Jim Mallinder's team-talk did the trick as they edged ahead ten minutes into the second half.
Tonga'uiha lunged over the line from short range after good pressure from the forwards and Myler added the conversion and yet another three points.
Priestland did briefly restore the Scarlets' lead shortly afterwards before Myler added a further two penalties from his unerring right boot.
Both teams had chances as the game wore on but, after the Scarlets had spent a long period camped on the Saints' line and severely tested their defence, Foden dealt the crucial blow.
The England full-back intercepted a pass in his own twenty-two and returned it the length of the field to score. Replacement Ryan Lamb added the conversion with the last kick of the match to effectively knock the Welsh side out of this year's competition.
Pens: Priestland 4
Tries: Tonga'uiha, Foden
Cons: Myler 2
Pens: Myler 5
Yellow card: May (Saints - 2nd min - foul play)
Scarlets: 15 Liam Williams, 14 George North, 13 Scott Williams, 12 Jonathan Davies, 11 Viliame Iongi, 10 Rhys Priestland, 9 Gareth Davies, 8 Ben Morgan, 7 Rob McCusker, 6 Aaron Shingler, 5 Dominic Day, 4 Lou Reed, 3 Deacon Manu, 2 Matthew Rees, 1 Iestyn Thomas.
16 Ken Owens, 17 Phil John, 18 Rhodri Jones, 19 Josh Turnbull, 20 Jon Edwards, 21 Rhodri Williams, 22 Stephen Jones, 23 Gareth Maule,
Northampton: 15 Ben Foden, 14 Chris Ashton, 13 George Pisi, 12 Tom May, 11 Vas Artemyev, 10 Steve Myler, 9 Martin Roberts, 8 Roger Wilson, 7 Phil Dowson, 6 Calum Clark, 5 Mark Sorenson, 4 Samu Manoa, 3 Brian Mujati, 2 Dylan Hartley, 1 Soane Tonga'uiha.
Replacements: 16 Mikey Haywood, 17 Alex Waller, 18 Paul Doran-Jones, 19 Christian Day, 20 Ben Nutley, 21 Lee Dickson, 22 Ryan Lamb, 23 Scott Armstrong
Venue: Parc y Scarlets
Referee: Alain Rolland (Ireland)
Assistant referees: Leo Colgan (Ireland), Michael Black (Ireland)
Television match official: Sean Flannery (Ireland)