Speaking in the wake of his Appeal Hearing in Glasgow against his initial 12 month ban from the game for feigning injury in that game, Williams said he "sincerely regretted" his role in the incident.
The Appeal committee decided to reduce Williams' ban to four months following his admission of guilt and introduction of new evidence in the case. He will now be able to play again after 19 November.
"I let down my team-mates and the club's fans, and I'll have to live with those actions for the rest of my career," said Williams
"I sincerely regret the role that I've played in this unacceptable incident that has done so much damage to the image of rugby union.
"In deciding to come clean and do the right thing, I've tried honestly to rectify this mess and repay the good faith shown in me by my friends and family.
"I would like to thank the ERC disciplinary panel for their decision to reduce my sanction and I am relieved that they did this. However, I also realise the grave error of judgment that I have made.
As a result of the new evidence submitted by Williams on the incident, the Appeal committee raised the sanction against Harlequins from Euros 250,000 to Euros 300,000, all of which is payable by 1 November.
They also imposed a three year ban from all ERC tournaments on the former Harlequins director of rugby, Dean Richards, and applied a two-year ban to the Harlequins physio Steph Brennan.
In a statement on their club website, Harlequins said they hoped they could now draw a line under the whole affair.
"Whilst the club are pleased that Tom Williams' ban has been reduced, we feel this is a very significant penalty in terms of fine," said the statement.
"We will now continue with our internal review into all aspects of the business to ensure that we have the most stringent compliance and robust policies and processes in place throughout the club and work towards rebuilding our reputation on and off the field.
"We hope that the club can now draw a line under what has been a difficult few months, learn from it and move on."