Leo Cullen’s men were left to rue a nightmare start which saw them fall 15-0 behind with their lineout malfunctioning and captain Isa Nacewa in the sin-bin. Clermont took their chances well, Peceli Yato and former England winger David Strettle scoring their tries.

But after the break, Leinster, who beat Clermont on the way to their last European Cup final appearance in 2012, came roaring back into the contest. Four penalties from the reliable boot of Jonathan Sexton cut the 15-3 interval deficit to a mere three points.

The game’s crucial moment came when a potential 56th-minute try for livewire flanker Dan Leavy was ruled out for holding at a ruck which was confirmed by TMO Jonathan Mason and gave referee Nigel Owens no option other than to rule out the score. Had the try counted, Leinster would have hit the front, but instead Morgan Parra kicked the resulting penalty for an 18-12 lead.

After that came two sublime drop goals from man-of-the-match Camille Lopez and that was enough to get the Top 14 side over the finish line, even though Garry Ringrose ran 60 metres to score a sensational try that Sexton converted. A late penalty from Sexton cut the gap to 27-22 but Clermont doggedly held on to set up a May 13 decider against Saracens in Edinburgh.

Giving his reaction afterwards, Cullen said of that critical penalty against Leavy: “(Dan) probably tries to stick (Aurelien) Rougerie but holds on too long. It’s one of those 50-50 calls. Unfortunately, it didn’t go our way but across the game there’s tonnes of those calls.

“It was a big turning point – 10 points – but even after that the players fought back well, created some more good opportunities, getting very, very close. We just couldn’t quite get our noses in front.

“If we do get our noses in front the game takes on a very different complexion with Clermont chasing the game and us adapting our game slightly. You can’t fault the effort of the players.

“There’s a lot of things in our control we could do better but overall we are very proud of the effort put in. We’re gutted to be out of the tournament but we’ll look back at the game and there is a number of things we can certainly do better and we have some big games coming up.

“It’s important we learn the lessons of how we manage the game because we have two games left in the PRO12 and then we will hopefully be involved in a semi-final and final after that.”

Cullen, senior coach Stuart Lancaster and the rest of the Leinster management will have high hopes for the remaining weeks of the season and beyond, with 11 players in today’s matchday squad aged between 21 and 24.

Yato opened the scoring, the Fijian flanker racing onto a Strettle chip to cross in the right corner. Parra converted and added a penalty and with Nacewa yellow carded for tugging back Strettle as he chased a Scott Spedding kick, the GUINNESS PRO12 leaders really had it all to do.

With a man advantage, Parra’s flat pass then found Strettle who sped past Leavy to add Clermont’s second touchdown with Joey Carbery unable to stop him. Parra’s conversion went wide, but the French club were utterly dominant.

Leinster’s failure to do the basics was letting them down, their lineout was particularly poor and contributed to giving Clermont the field position from which Strettle went over. Cullen’s side, whose sloppiness and defensive frailties had contributed to Clermont’s super-charged start, were in real trouble, but they did rally and after Parra put a penalty wide, Sexton finally got the province on the board with the last kick of the first half.

The second period started as the first ended with Sexton splitting the posts with his second penalty success and Leinster were now playing with much more aggression, accuracy and tempo. Workhorse flanker Rhys Ruddock was leading by example up front.

The visitors, who showed greater composure and buckets of character during the closing 40 minutes, were suddenly transformed and with Clermont tiring in the French sun, Ringrose went close to a try following some sustained possession.

There was no score, but two more place-kicks from Sexton meant there were now just three points (15-12) between the teams. Leinster then thought they had got their first try, Fergus McFadden leading a brilliant breakout from which Leavy made it to the line to dot down.

Leinster had to really chase the game after the Leavy try was ruled out and Parra duly landed his second penalty, as the momentum swung back to les Jaunards. Ringrose gave the province a lifeline with his wonder try, showing great feet in midfield and using his pace to make the line from halfway.

However, the experienced Clermont outfit shut the door on Leinster’s courageous comeback as key man Lopez struck a 72nd minute penalty and a second drop goal before Sexton’s fifth penalty of the day was the final scoring act of a breathless and hugely entertaining semi-final.