The ex-Lions tourist, who had been director of rugby at the London club for the past ??? seasons, is undecided as to whether to appeal the length of his suspension but has insisted the blame lies squarely on his own shoulders.
"I have always been passionate about rugby and in the dying minutes of that Heineken Cup quarter-final - a stage Harlequins had never reached before - I allowed my passion for the club to compromise my judgment," said the 46-year-old.
"I ordered a blood substitution to be made using fake blood. Thankfully the substitution did not affect the outcome of the game.
"I have since made clear that I accept full responsibility for what happened and have apologised for my actions. I honestly believed that I was acting in the best interests of the club and my colleagues, a clearly mistaken belief.
"I am mortified that this matter has brought damaging publicity on Harlequins, members of its staff, my own family and the wider game of rugby."
Richards did admit, however, that he was unaware of the fact that the substituted player, Tom Williams, had had the inside of his mouth deliberately cut after the incident in order to make the situation appear more realistic if any checks were made.
"Some eight days later I was told by Tom Williams that, in the heat of the moment and at his direction, he had his lip cut to make it appear as if there had been a real injury if anyone asked to look at it," explined Richards.
"Believing this to be true, I felt a managerial duty and sense of loyalty to safeguard the professional position of those involved.
"From then on, with the agreement of Tom Williams, I suggested the fabrication of a story which led to us all withholding the true facts, a position that was obviously wrong."