Chief Whip Andrew Mitchell's political career is hanging in the balance as he faces calls to quit after being accused of calling No 10 police officers "plebs".
Police suggested the Tory enforcer was close to being arrested after allegedly shouting "learn your f****** place" when he was stopped from cycling out of Downing Street's main gates.
Although the former shadow police minister has apologised for not treating officers with due respect he has denied using some of the language reported.
Labour said Mr Mitchell was either accusing the member of Scotland Yard's Diplomatic Protection Group, SO6, of lying or he must have used the foul language, potentially an arrestable offence.
John Tully, chairman of the Metropolitan Police Federation, said Mr Mitchell was warned to curb his behaviour by officers at the time of the rant and was likely to have been arrested if he had continued.
"There is a written record of the incident in the officers' notebooks," he told Channel 4 News. "I understand that following his tirade to the officers he was warned and he desisted. I think had he continued he would have been arrested."
He added: "He should resign. As a Cabinet minister it's unacceptable for someone of his standing to use such disrespectful and abusive language to a police constable let alone anyone else."
David Cameron dodged questions about whether he planned to sack Mr Mitchell but condemned his behaviour as "wrong" and "inappropriate".
Shadow home secretary Yvette Cooper dubbed the reported outburst an "utter disgrace", and questioned how Mr Mitchell could do the job of chief whip if he could not keep his temper in check. "These are appalling reports. No one should treat police officers or public servants in this way," she said.
Mayor of London Boris Johnson was said to be "deeply unimpressed" with Mr Mitchell's behaviour, while former Conservative Party chairman Lord Tebbit denounced the comments as "extraordinarily stupid" and urged Mr Cameron to have a "heart-to-heart talk" with his new chief whip.