The 'plebgate' row that engulfed Andrew Mitchell was sparked by a police officer who posed as a member of the public and falsely claimed to have witnessed the events, it has been said.
The officer allegedly wrote to his local MP giving details of the Chief Whip's behaviour when he was blocked from cycling out of the main gates in Downing Street.
Number 10 said the claims - which emerged after a member of the diplomatic protection squad was arrested - were "extremely serious".
"Any allegations that a serving police officer posed as a member of the public and fabricated evidence against a Cabinet Minister are exceptionally serious," a spokesman said. "It is therefore essential that the police get to the bottom of this as a matter of urgency."
However, Metropolitan Police Commissioner Bernard Hogan-Howe has insisted he has seen nothing to undermine the official account of officers who were there.
The developments came as Channel 4 News showed previously unreleased footage of Mr Mitchell from the night of the incident in September.
Although there is no sound, the MP - who clung on to his job for a month before finally resigning - can be seen with his bicycle talking to three officers by the main gate for around 20 seconds.
He then wheels it over to the side gate and exits through it. Footage from other cameras suggest that there were few members of the public passing by at the time - apparently contradicting the leaked police log.
Mr Mitchell accepts that one of his parting shots to the on-duty officer was: "I thought you guys were supposed to f****** help us."
But he denies calling him a "pleb" or a "moron", insisting parts of a police log of the incident published in the media were "false".