New Magners League champions Munster lost for the first time in eight matches in the competition as a late converted try from Ceri Sweeney copperfastened a 20-12 home win for the Cardiff Blues and their automatic qualification for next season’s Heineken Cup.

Although Cardiff had reason to celebrate after confirming their place in next season’s Heineken Cup, a shoulder injury sustained by bustling centre Jamie Roberts did give them cause for concern.

Coming just a few days after Tom Shanklin dislocated his right shoulder, the British & Irish Lions management will be nervously awaiting an update on Roberts’ condition in the coming days.

Encouragingly though, it did not look too serious as he made his way off early in the second half and his withdrawal was more precautionary than anything else.

There were five 2009 Lions squad members on the pitch at the Arms Park, as both Cardiff and Munster looked to bounce back from disappointing Heineken Cup exits last weekend.

Munster included Keith Earls and Donncha O’Callaghan for their first match since losing to arch rivals Leinster at Croke Park, while Andy Powell, Roberts and Martyn Williams, who missed that sudden death penalty kick against Leicester seven days ago, lined out for the Blues.

Cardiff boss Dai Young originally selected another Lion, Leigh Halfpenny, for this game but he had to pull out before kick-off due to a hamstring twinge. Tom James indirectly replaced him, with Chris Czekaj moving to full-back.

Munster were without inspirational captain Paul O’Connell, who rested a back injury, and they were also forced into a late team change with Brian Carney opting out due to a family bereavement and Kieran Lewis replacing him on the right wing.

The Blues have not lost at home to an Irish side since September 2007 and they made a strong start to what was a free-flowing game, albeit with neither side able to be clinical in the opening half.

Out-half Nicky Robinson missed two kickable penalties, before spurning a third opportunity and going for position near the Munster try-line.

It paid off, superb handling along the line – particularly by Czekaj – let Tom James trot in on the left for the first try.

Munster, a man short after flanker Nick Williams was sin-binned for a late tackle on Roberts, were under pressure for long periods.

But they broke downfield, with full-back Denis Hurley leading the way, and battered at the home line until number 8 Denis Leamy stretched over to score from close range.

Robinson finally landed a penalty in first half injury-time, but the 8-5 interval margin did not reflect the fact that the Blues had had the better of the play.

Early in the second half, the home side lost Roberts and then almost conceded a try when impressive flanker Niall Ronan raced clear, but a knock-on ruined Munster’s chances of scoring.

With Barry Murphy, Ronan and Williams carrying well, the Irish province were showing greater urgency and the Blues needed television match official Neil Ballard to rule out a Leamy effort.

Then centre Murphy were held up over the line, with Martyn Williams brilliantly rescuing his side.

However, Munster’s efforts were eventually rewarded when the ball was spun wide to the right, Earls found some extra space and winger Kieran Lewis crossed in the corner with Jeremy Manning coolly landing the conversion for 12-8.

Cardiff gradually worked their way into the Munster 22 for the first time in the second half, and wasted little time in striking for a timely try.

Munster were pressured at a scrum, lost the put-in and Sam Warburton broke from the back. Robinson and James were both involved before scrum half Darren Allinson got close to the whitewash and then a quick switch to the right found powerful prop Gary Powell who gleefully crashed over.

Hurley had to track back and prevent Nicky Robinson from claiming a third try off a Williams kick through, and Munster were being pinned back in their own half for the closing stages.

They brought on reinforcements, including hooker Mike Essex who played every minute of Shannon’s extra-time AIB League final win over Clontarf on Saturday.

Ireland Under-20 out-half Andrew Burke also came on to make his senior debut for the province but it will be a game he will long to forget.

With his first touch of the ball on 79 minutes, a Burke pass was gobbled up by Cardiff replacement Ceri Sweeney who raced clear and in under the posts for the clinching try, which Robinson converted.

Speaking afterwards, Munster coach Tony McGahan said: "It’s always difficult to play after such an emotional week as we have had. So I thought this was actually a pretty brave performance considering how little possession we had against a good team.

"We were under a lot of pressure in the first half, especially in the scrum which is something we will have to have a long look at.

"But we were far more tenacious in the second half, we had a lot more field position and had opportunities to score.

"It was a tough break for Andrew at the end but that’s professional rugby. That mistake didn’t cost us the win.

"We had 79 minutes before that to win the game and we didn’t. But he will learn from experiences like that."

An obviously pleased Dai Young said: "We’ve enjoyed the Heineken Cup this season and wanted to make sure we’re there next season and to do it with two games in hand is pleasing.

"Large parts of our game were a typical end-of-season game, a bit sloppy, our execution and skills weren’t great out there.

"We’re at a stage now where there’s a lot of tired bodies, both mentally and physically and I think everyone just wants to get over that last hurdle of the last couple of games."