Chelsea were fined 40,000 pounds by the English Football Association on Wednesday for breaching the governing body's doping regulations.
The English Premiership champions had denied the charge but an Independent Disciplinary Commission ruled the club should be fined and issued with a warning for carrying out private drug testing, a breach of FA rule E25, on some of its' players on 12 July 2004.
A statement on the FA's website confirmed: "Chelsea were charged in relation to the independent private drug testing the club conducted on some of its players on 12 July 2004.
"The Commission found that, in conducting such tests, the club had breached The FA's Doping Control Regulations."
Chelsea were furious with the punishment, which they said related to tests they carried out on striker Adrian Mutu.
Romania star Mutu, who now plays for Italian club Fiorentina, was sacked by Chelsea in 2004 after testing positive for cocaine.
A statement on Chelsea's website said: "While we accept there was a technical breach of the doping regulations in this case, we find it extraordinary that we have been found guilty of trying to undermine the drug testing system.
"Chelsea FC would never contravene any rules in order to cover up drugs in sport. These rules are in place to punish those who seek to protect drug takers.
"Our stance in the Adrian Mutu case proves that our sole intention in conducting a private test was to protect football from a drug taker.
"Firstly, we had suspicions that this player was a regular user of a Class A drug and the FA random drug test procedures had failed to confirm this.
"Secondly, the player was not privately tested for performance enhancing drugs, only for recreational drugs.
"In the circumstances, the test on July 12, 2004 proved negative and an FA approved test was requested later that summer which was positive.
"The player's contract was then terminated and during the hearing into this failed test he also admitted to taking cocaine on at least seven occasions, vindicating our actions."