Scotland Yard was condemned last night for arresting a police officer on suspicion of leaking details of Andrew Mitchell's angry outburst when he was stopped from using Downing Street's main gates.
The Police Federation expressed "great regret" over the decision, while a leading London Assembly member suggested there could have been "political pressure" behind the move.
The officer, understood to be from the diplomatic protection group SO6, was arrested in connection with the leak about the fracas, which led to Mr Mitchell's resignation as Chief Whip in October.
According to accounts of the police log, Mr Mitchell allegedly described an officer who stopped him from wheeling his bicycle out of the gates as either a "pleb" or a "moron". On quitting, Mr Mitchell insisted in a letter to David Cameron that he had not used either word, but admitted delivering the parting line: "I thought you guys were supposed to f****** help us."
Mr Mitchell said yesterday: "It's the contents of the alleged police log which are false… they are false and I want to make that very clear."
John Tully, the chair of the Met Police Federation, said: "It is of great regret that the [Met] has found it necessary to take this action. I trust justice will ultimately prevail."
The Independent Police Complaints Commission last night said it was supervising the investigation into the "validity" of an officer's claim that he witnessed the row involving Mr Mitchell.