The "plebgate" row that engulfed Andrew Mitchell was allegedly fuelled by a police officer who posed as a member of the public and falsely claimed to have witnessed the events.
The officer is said to have written 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 "exceptionally 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.
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"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 aired previously unreleased CCTV footage of Mr Mitchell from the night of the incident in September.
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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. Footage from other cameras suggests that there were few members of the public passing by at the time – apparently contradicting a leaked police log.
In his first full interview since the episode, Mitchell demanded a full probe into the police account of events and insisted the email was key to the loss of his job.
"I always knew that the emails were false, although extremely convincing," he told Channel 4 News last night. "It has shaken my lifelong support and confidence in the police.
"I believe now there should be a full inquiry so we can get to the bottom of this."
In the missive to his MP John Randall – who was also Mitchell's deputy in the government whips' office – the police officer apparently did not disclose his job.
He described how he had been walking past Downing Street with his nephew when the spat took place.
The email said Mitchell had sworn repeatedly and called the officers on guard "plebs". It also suggested that passers-by outside the gates had been shocked, and some may have filmed the confrontation.
The account closely matched that in the official police log, which was later leaked to the Daily Telegraph.
Channel 4 News said David Cameron had called Mr Mitchell after being informed of the email's existence, and suggested he had been "caught bang to rights".
When his Chief Whip flatly denied key parts, the Prime Minister ordered Cabinet Secretary Jeremy Heywood to carry out an investigation.
However, his probe failed to discover more about the identity of the emailer – despite Randall going round to see the man in person.
Cameron apparently only learned over the weekend that the "witness" was a serving police officer, and was said to be "furious".
When called by the programme, the unnamed individual seemed to admit that he had not been present when the row happened.
The CCTV also showed no evidence of passers-by who could have been a man with his nephew.
The situation is further complicated by speculation of tensions between Mitchell and Randall – who was reportedly ready to quit unless his boss left his post.
Mitchell said his first reaction to the story emerging in September was "there's not really much of an altercation".
"There were three phrases above all which were hung around my neck for the following 28 days every day in the press which were used to destroy my political career and were used to toxify the Conservative Party," he said.
"They are completely untrue, I never said phrases like that at all, I would never call someone a f***ing pleb, anyone who knows me well would know that it is absolutely not in me to use phrases like that."
Asked why he did not give a more detailed account before, Mitchell said: "Well when the story broke, the decision was made that I would apologise for what I did say, and my apology was accepted, there was no police complaints and that we would let it lie.
"Now with the benefit of hindsight, that was clearly the wrong decision."
Mayor of London Boris Johnson said: "These are very serious allegations that must be investigated with all possible urgency.
"An allegation that a serving police officer posed as a member of the public whilst fabricating evidence is a matter of the utmost gravity.
"I know that the Metropolitan Police Commissioner is committed to establishing the truth here, as soon as possible."
But, speaking before the programme went out, Hogan-Howe told London's LBC 97.3 radio: "I don't think, in terms of what I've heard up to now, that it's really affected the original account of the officers at the scene.
"Because of course this officer we've arrested wasn't any of those people involved originally. This is another officer who wasn't there at the time."
In a separate BBC interview, he said: "There is more to this than meets the eye. I am afraid I am constrained in explaining that and I hope that when people hear the full story they will support what we've done.
"We got some new information, we acted on it quickly and I hope, in time, when we are able to explain the sequence of events, people understand why we did what we did."
A spokeswoman for The Police Federation of England and Wales said: "We are aware that there is an ongoing investigation into this matter, we are therefore unable to comment further at this stage."
Former Tory leader Lord Howard told BBC Radio 4's Today: "I was very sad when Andrew Mitchell resigned because I think he's an excellent minister.
"I am appalled at the suggestion that a police officer could do what it is alleged that the police officer in question did.
"And I certainly do hope that Andrew Mitchell is restored to government at the earliest possible opportunity."
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