Labour has asked Downing Street's most senior civil servant to investigate Andrew Mitchell's angry tirade at Downing Street police officers, claiming the Chief Whip's account of the row is "unravelling by the day".
Cabinet Secretary Sir Jeremy Heywood must also release, in full, accounts of what was said during the outburst, shadow home secretary Yvette Cooper said.
Deputy Prime Minister Nick Clegg has suggested the Tory enforcer should be allowed to "draw a line" under the incident while Tory Communities Secretary Eric Pickles said his colleague should not be sacked for having a "bad temper".
But Labour insisted Prime Minister David Cameron could not "dismiss" the incident after reports that Mr Mitchell admits to swearing when officers refused to let him cycle out of Downing Street's main gates.
Ms Cooper has written to Sir Jeremy asking for Mr Mitchell and the officers involved to be interviewed, statements to be reviewed and an examination of CCTV footage.
A swift investigation would allow Mr Cameron to "decide on the Chief Whip's continued position" based on the facts, she added.
"We now know that Andrew Mitchell swore at police officers who were doing their job protecting Downing Street," Ms Cooper said.
"This is completely unacceptable behaviour from a Cabinet Minister, and a half-hearted private apology is clearly insufficient."
The Chief Whip admits swearing at No 10 police officers but insists he did not call them "plebs", according to The Sunday Telegraph.
It suggests Mr Mitchell said: "Look, I'm the chief whip, I work at Number 9 (Downing Street)," before muttering: "You guys are supposed to f****** help us."