Chelsea deliberately targeted Adrian Mutu because they suspected him of using drugs, chief executive Peter Kenyon has told BBC Sport.
The club tested Mutu, who was sacked on Friday, because he missed training and his performances were below par.
"A clinical assessment over time led us to believe that his behaviour could be associated with drugs," said Kenyon.
"The contract is quite explicit that taking any form of drugs is gross misconduct and will lead to dismissal."
Kenyon added: "We believe in zero tolerance - the manager has to have the confidence of his players.
"We've got to look after the interests of Chelsea and it's important to make a statement on a very important issue in sport.
"Once the admission of guilt was determined then it was clear what our decision needed to be.
"We are his employer and it's under contract law at that point - there is no doubt of Adrian's guilt in taking cocaine."
But Professional Footballers' Association boss Gordon Taylor has hit out at Chelsea's treatment of Mutu.
"They have target-tested the player with a view to getting rid of him," said Taylor.
"The attitude may be zero tolerance but it's not a policy we would approve of.
"Chelsea have a duty of care. We would expect an interest in the moral and social welfare of its employees."
Chelsea coach Jose Mourinho said: "We did target Mutu for a test, but only because we got some signals over the weeks.
"Over the weeks we had some question marks, and because of those question marks felt it was important to target a drug test."
He also hit back at PFA chief Taylor, saying: "He is paid to do his job well, but he knows nothing about what goes on at Chelsea."
Mutu's contract was terminated by Chelsea on Friday morning, with the London club stating "any player who takes drugs breaches his contract with the club".
The Romanian international will pay a heavy price for his use of cocaine - while Chelsea will write off the £15.8m they paid Parma for him last season.
Taylor is shocked at Chelsea's decision, saying: "I find it astonishing they are not prepared to discuss the situation with him and set an example, encourage rehabilitation and get him back on track.
"Quite a number of other clubs have been prepared to engage in rehabilitation of the player."
And Taylor also believes Chelsea discouraged their staff from attending a drugs advice session run by former Arsenal player and alcoholic Tony Adams' Sporting Chance organisation.
Taylor said: "Sporting Chance personnel go to the club in order to talk and inform the players, but Chelsea chose to give the first team and the second team players the day off."
The PFA has been involved in Mutu's case since his failed drugs test came to light and Taylor said they would continue to assist.
"At the moment there has got to be a formal hearing with the FA and then there is the Chelsea situation," he added.
Arsenal boss Arsene Wenger appeared to back Chelsea's stance.
"I do not know how guilty Mutu is but I agree with the high moral standards at the club," said the Gunners boss.
"Players and sportsmen have a big responsibility and so therefore you cannot
be against a decision like that.
"You have to analyse these things case by case. We had a case of Merson who
came out and said 'listen, I'm cocaine addicted, I want help'. So if a player
comes out like that, can you just sack him?
"I feel if he wants help, you have to help him. But, in the case of Mutu, there was a split anyway before the drug problem happened. So maybe it was in the best interests of the two (Chelsea and Mutu) that they split."