Cardiff Blues produced an impressive 39-16 derby performance to beat the Scarlets, which kept alive their very slim hopes of making the play-offs.
British & Irish Lions star Jamie Roberts picked up a brace against the side from Llanelli, who leaked 24 unanswered points in the second half and will now have to hope the Blues win the Amlin Challenge Cup in order to gain a back-door entry into Europe's premier club competition.
Second-half tries from Taufa'au Filise, Roberts and Richie Rees killed off the Scarlets in emphatic fashion.
The Scarlets went into the match boosted by Stephen Jones extending his contract until the end of the 2011-12 season.
But they were without another Lion in Matthew Rees, who was sidelined with a neck injury.
The Blues were missing Andy Powell, Sam Norton-Knight, Tom James, Gareth Cooper, Tom Shanklin and Gary Powell.
The Scarlets just edged an enthralling, high-tempo first period 16-15 despite trailing two tries to one.
First blood was drawn by the Scarlets after 11 minutes when centre Regan King weaved his way over after appearing on the shoulder of Stephen Jones for the opening try.
But the home side's 7-0 advantage was wiped out in the space of five minutes, first by a Ben Blair penalty and then by a try for Jamie Roberts who was sent over in the left corner for a converted try from a neat pass out of the tackle from Casey Laulala.
However, a penalty goal and then a drop-goal from Stephen Jones gave the Scarlets a 13-10 advantage.
The Blues probably thought they had the interval lead through a try for lock Deiniol Jones who stretched for the line after another dash down the left by Roberts.
But from the restart hooker Gareth Williams was penalised for obstruction and Jones kicked his second penalty to give the Parc y Scarlets outfit a one-point half-time lead.
Two tries in seven minutes at the start of the second half finished off the Scarlets and confirmed the Blues' place in the Heineken Cup for next season.
The writing was on the wall for the Scarlets when prop Filise crashed over after just two minutes.
And the Blues confirmed a bonus point when Roberts breezed in for his second try after Martyn Williams won ball on the floor and it was put through the hands with Xavier Rush twice involved.
The final nail was put in the Scarlets' coffin when scrum-half Rees went over for his side's fifth try on 58 minutes to stretch the lead to 20 points.
With the Scarlets chasing the game the last say was left to the Blues who registered the match's final score with a penalty goal for Blair in the final minute.
Pen: Jones 2
For Cardiff Blues:
Tries: Roberts 2, Jones, Filise, Rees
Con: Blair 4
Pen: Blair 2
Scarlets: 15 Daniel Evans, 14 Sean Lamont, 13 Regan King, 12 Jonathan Davies 11 Andy Fenby, 10 Stephen Jones, 9 Tavis Knoyle, 8 David Lyons (capt), 7 Richie Pugh, 6 Josh Turnbull, 5 Dominic Day, 4 Lou Reed, 3 Rhys Thomas, 2 Ken Owens, 1 Phil John.
Replacements: 16 Deacon Manu, 17 Emyr Phillips, 18 Damian Welch 19 Rob McCusker, 20 Martin Roberts, 21 Rhys Priestland, 22 Lee Williams.
Cardiff Blues: 15 Ben Blair, 14 Leigh Halfpenny, 13 Casey Laulala, 12 Jamie Roberts, 11 Chris Czekaj, 10 Ceri Sweeney, 9 Richie Rees, 8 Xavier Rush, 7 Martyn Williams, 6 Maama Molitika, 5 Paul Tito (capt), 4 Deiniol Jones, 3 Taufa'au Filise, 2 Gareth Williams, 1 Gethin Jenkins.
Replacements: 16 T Rhys Thomas, 17 John Yapp, 18 Bradley Davies, 19 Sam Warburton, 20 Lloyd Williams, 21 Dai Flanagan, 22 Dafydd Hewitt.
Referee: Nigel Owens (WRU)
Assistant Referees: Phil Fear (WRU), Huw Griffiths (WRU)
TMO: Tony Rowlands (WRU)