Government chief whip Andrew Mitchell has apologised for reportedly launching a tirade at police officers guarding Downing Street.
The Cabinet member said he "did not treat the police with the respect they deserve" but denies using language quoted in the Sun on Friday.
According to the newspaper, the Conservative MP shouted abuse at officers because they would not let him cycle out of Downing Street on Wednesday. He reportedly said: "Best you learn your f***ing place. You don't run this f***ing government.
The Sun added that Mitchell also called the officers "morons" and, most damagingly, told them: “you’re f***ing plebs".
Labour's shadow home secretary Yvette Cooper said the allegations were "appalling" if true and police officers and public servants should be treated better by ministers.
"And the idea that a Cabinet Minister could behave like this towards police officers doing their job is an utter disgrace," she said.
"It tells you all you need to know about this out of touch government and its attitude to public servants.
She added: "How can a Chief Whip hope to do his job or instill respect when he behaves like this and can't even keep his cool?"
Earlier, shadow defence secretary Jim Murphy tweeted: "Some of these Tories are foul-mouthed spoilt little brats".
Mitchell's alleged outburst came just days after police officers Fiona Bone and Nicola Hughes were shot dead in Manchester.
Prime Minister David Cameron is aware of the outburst, "is glad that Mitchell has apologised" and said "police should always be treated with respect", according to a spokeswoman for No 10.
The MP for Sutton Coldfield apologised in a statement, which read: "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."
Mitchell was appointed to the role of chief whip in this month's Cabinet reshuffle, having previously served as International Development Secretary.
The former army officer know as 'Thrasher' was promoted to bring greater discipline to the Tory troops in the wake of a series of rebellions and is also reported to have told police: "Open this gate, I’m the Chief Whip. I’m telling you — I’m the Chief Whip and I’m coming through these gates."
While the alleged use of blue language is embarrassing for the minister, it is arguably the use of the word "pleb" that is most damaging for the government.
Eton-educated David Cameron is sensitive to attacks on his upper class roots and has often been criticised for being out of touch with ordinary people.
In April Tory MP Nadine Dorries attacked the prime minister and George Osborne for being "two arrogant posh boys" with "no passion to want to understand the lives of others".