Ali Williams© Photo by David Rogers/Getty Images

Dai Young said Toulon match-winner Ali Williams escaped a "blatant yellow card" before scoring the try that sealed Wasps' Champions Cup 32-18 quarter-final defeat.

Rugby director Young admitted Wasps were left frustrated by former All Blacks lock Williams escaping a sin-binning for tugging back Elliot Daly off the ball. Wasps closed to within seven points of double European champions Toulon with six minutes to play thanks to wing Will Helu's second try, but Williams crossed in the dying stages to close out victory.

Superstar-studded Toulon struggled through unforced sloppiness throughout, before Williams sealed the triumph - and Young is adamant the 33-year-old should have been off the field at the time of that pivotal score.

"There are three people who didn't see it - the referee and the two touch judges," said Young. "I don't want it to sound like sour grapes because they have probably had a bit too much for us on the day. But in saying that we ended up scoring three-and-a-half minutes later.

"So if we'd had a penalty try then and the yellow card, it would probably have allowed us to really mount a real challenge. It certainly would have made it interesting.

"It's frustrating because if you look at their first try, their lineout in the build-up was nowhere near straight. Little things go against you but when you've won the European cup twice you tend to get the 50-50s.

"We were hugely frustrated by that, hugely frustrated by the decision and them still having 15 players on the field when it was a blatant yellow card and a blatant sin-bin."

Wasps boss Young admitted Aviva Premiership clubs naturally face an uphill struggle against their cash-rich French counterparts, whose squad spending knows precious few limits. Premiership champions and runaway league leaders Northampton fell well short in a 37-5 defeat to Clermont Auvergne, leaving Saracens as England's sole semi-final representatives.

Young admitted the top Premiership sides only stand a chance of seeing off the cream of the French crop when they can field their strongest line-ups.

"The top four teams consistently over the last three or four seasons are up there," said Young. "But you know you've got to catch them on a day when they are having problems, because there are world-class players across the board.

"There's no room for error against these top French teams. Their weakest players tend to have only 25 caps as opposed to 45. So they have very few weaknesses right across the board and I think when Leigh Halfpenny had his bump on the head and he was ruled out, it left us only five international full-backs to look at really! It's certainly a step up when you know what you're coming up against.

"Teams in the Premiership must arrive at these games playing their best team. There's no way we can match their squads with the salary cap, and two or three injuries will obviously hurt us a lot more than it will for Racing, Clermont and Toulon."