Their first ever trip to Stade Pierre Antoine turned into a nightmare as, despite outscoring Castres by two tries to none thanks to scores from Girvan Dempsey and Jonathan Sexton, Leinster were overhauled during a hard-fought second half.

Inspired by battling back rowers Florian Faure and Chris Masoe, returning captain Lionel Nallet and a vocal crowd, Castres clawed their way to victory.

Man-of-the-match Anthony Lagardere's haul of six successful penalty goals was enough to seal only their fourth win in 16 matches this season.

The result means that should Wasps muster a bonus-point victory over Edinburgh at Adams Park on Sunday, Leinster's Pool Two lead will be down to just two points and the clash between Wasps and Leinster at Twickenham on January 17 will effectively become a winner-takes-all contest.

Reflecting on his side's collapse to Castres, Cheika said: "We gave away too many free-kicks, we didn't play well at the ruck and they were pouring numbers in at the ruck.

"We needed to commit more numbers at the ruck and we got penalised heavily at that area and it cost us.

"We didn't play well tonight, we didn't have nearly enough field position and we didn't kick very well. We took our tries well though.

“It's a shame the winning run is at an end but now we cannot afford to have another loss, so (the Wasps game at Twickenham) is a straight shoot-out. It's a final almost."

The reigning Magners League champions scored a superb 41-11 win over Wasps at the RDS in October but they have struggled for consistency since then and Cheika will be demanding a rapid improvement from his players.

"We've got to take charge the next time (the Heineken Cup) comes around...get better and more solid in the pressure moments.

"We've got to get back to the level we showed in the first two rounds in Europe, and make sure by the time that we get to that Wasps game we're in a really competitive state because that's a really important match."

Former England international Phil Christophers, who played on the left wing for Castres, praised the way his team fought back from 15-9 down to end their recent run of defeats.

Christophers said: "The first 20 minutes there was pressure on us and we weren't playing to our strengths.

"But in the second half, we were more aggressive, we tackled and tackled and we got the crowd going with our play.

"We adopted a simple game-plan, played the game in Leinster's half and I could feel that they were doubting themselves a little bit.

"Once we put the pressure on, they put in a few bad kicks and we capitalised on their mistakes.

"We've lost five home games this season, so it's great to get back on the winning side."