Williams was found to have faked a cut to his mouth, which allowed Harlequins to send specialist goal-kicker Nick Evans back onto the field with five minutes remaining and Leinster leading 9-6.

Harlequins director of rugby Dean Richards plus two members of the club's medical team - physiotherapist Steph Brennan and Dr Wendy Chapman - both had misconduct complaints dismissed.

Although Harlequins avoided being thrown out of the tournament and had 50% of the fine suspended for two years, the club were hit hard when the verdict was delivered.

A club statement said: "Harlequins are both surprised and disappointed at today's decision by the ERC disciplinary committee to find Harlequins and Tom Williams guilty of misconduct - particularly so in the light of the acquittal of Dean Richards, Steph Brennan and Dr Wendy Chapman on similar or identical charges.

"The club and the player will consider their position in the light of the written judgement due to be handed down by the disciplinary committee."

Both Williams and Harlequins have the right to appeal.

An investigation was launched into the circumstances surrounding Williams' replacement late in a nailbiting clash at The Stoop.

Evans had been substituted earlier in the half but Williams' departure to the blood-bin allowed the New Zealand fly-half to return to the action.

Television cameras spotted Williams winking towards the bench with 'blood' smeared around his mouth.

Harlequins managed to engineer a late drop-goal attempt but Evans was struggling with a knee injury and pulled it wide.

Over the course of the three-day hearing, the disciplinary committee heard submissions from representatives of ERC and Harlequins and from a number of witnesses as well as considering further evidence, that included broadcast footage from the game.

The disciplinary verdict read: "It was the view of the committee that this was a very serious offence and one that damaged the reputation of the tournament and of rugby union, and accordingly the committee imposed a fine of 250,000 euros on Harlequins, of which 50% is suspended for two years.

"The committee also suspended Mr Williams from playing rugby for a period of 12 months up to and including 19, July 2010."

A one-year suspension would be a major blow for Williams, who has developed into a key member of the Harlequins squad.

Richards championed Williams' international credentials last season and believes he deserved at least a place in the England Saxons set-up.