Andrew Mitchell was facing calls to quit tonight as police insisted details of the chief whip's tirade against an officer were accurate.
The Tory enforcer was accused of shouting "learn your f****** place" after being stopped from cycling out of Downing Street's main gates and described officers as "plebs" and "morons", according to reports.
Although the former shadow police minister has apologised for not treating the police with proper 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: "The information I have is that Mr Mitchell did use offensive language - he used the words 'pleb' and 'morons' as well as expletives, which is totally unacceptable," he said.
"It constitutes a criminal offence under the Public Order Act, Section 5, and I would say he's very lucky not to have been arrested."
"I believe he should resign. If he chooses not to, then I believe the Prime Minister should sack him," added Mr Tully.
David Cameron dodged questions about whether he planned to sack Mr Mitchell and condemned his behaviour as "wrong" and "inappropriate".
The Prime Minister appeared tense when asked about the outburst during a visit to Greater Manchester Police headquarters to pay his respects to murdered PCs Fiona Bone and Nicola Hughes.
"What Andrew Mitchell said and what he did was not appropriate. It was wrong and it is right that he has apologised," he said.
"He has obviously apologised to me, but more importantly he has apologised thoroughly to the police and that needed to be done.
"Police do an outstanding job across our country. They do a very important job protecting places like Number 10 Downing Street and I am very conscious of the protection they give to me and my family and the work they do for everyone in public life.
"I am eternally grateful for that and the police should always have our respect and our help and support and that is very, very important."
Mr Mitchell made a personal apology to the officer involved by telephone this morning as he tried to limit the damage from the episode.
A Cabinet Office spokesman said the officer had accepted Mr Mitchell's apology. No formal complaint has been filed over the politician's behaviour.
The MP apparently pulled out of a speaking engagement in Reading as news of the row broke.
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" by Mr Mitchell 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.
The MP for Sutton Coldfield, who was also a minister under John Major in the early 1990s, apologised in a statement.
He said: "On Wednesday night I attempted to leave Downing Street via the main gate, something I have been allowed to do many times before.
"I was told that I was not allowed to leave that way. While I do not accept that I used any of the words that have been reported, I accept I did not treat the police with the respect they deserve."I have seen the supervising sergeant and apologised, and will also apologise to the police officer involved."
Senior Tory backbencher David Ruffley defended Mr Mitchell. "I have known Andrew Mitchell since he became shadow police minister, a post I subsequently held," he said.
"So I can attest to the fact that he has the highest regard for the police service of this country.
"As a strong supporter of tough, traditional law and order policies, Andrew understands the central importance of police officers in fighting crime and keeping us safe.
"Nothing that may have occurred this week should affect his well-deserved reputation for being on the side of policemen and policewomen."
Mr Mitchell, a keen cyclist, was reported by The Sun to also have called the police "morons".
"Best you learn your f****** place. You don't run this f****** Government. You're f****** plebs," it reported him as saying.
Earlier this year Scotland Yard Commissioner Bernard Hogan-Howe said there "were still opportunities" to arrest members of the public for swearing at police despite a judge ruling that it is not a crime.
Asked if Mr Johnson stood by comments he made last year that people who swear at police should expect to be arrested, the Mayor's official spokesman said: "Yes, he does."
Asked if the chief whip should remain in post, he added: "That is not a matter for the Mayor, it is a matter for the Prime Minister and Mr Mitchell."