Scotland Yard today vowed to "get to the truth" of allegations that a police officer falsely claimed to have witnessed the "plebgate" row that led to Andrew Mitchell being forced to quit as Tory Chief Whip.
The officer is said to have written to his local MP, posing as a member of the public, giving details of the altercation that took place when the senior Tory attempted to cycle 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".
The Metropolitan Police Service said today it was conducting a "thorough and well-resourced investigation" that could look at conspiracy.
In a statement it said: "The allegation that a serving police officer fabricated evidence is extremely serious.
"It goes to the very heart of the public's trust in the police service.
"The Metropolitan Police Service is conducting a thorough and well-resourced investigation to get to the truth of the matter as quickly as possible.
"Since receiving fresh information last Thursday, the MPS has acted swiftly.
"On Saturday, an investigation into the alleged unauthorised disclosure of information was reopened. On Saturday evening, an officer was arrested on suspicion of misconduct in public office.
"On Sunday, the investigation was referred to the IPCC ( Independent Police Complaints Commission) for supervision.
"The investigation is looking at the alleged unauthorised disclosure of information and the circumstances surrounding a police officer's claim to have witnessed an incident in Downing Street.
"If any evidence emerges of conspiracy this will form part of the investigation.
"This is a fast-moving and comprehensive investigation and the highly unusual events of the last three days have shown this inquiry will need to go where the evidence takes us as it progresses."
It comes after CCTV footage of the incident was broadcast for the first time and appeared to contradict parts of a leaked police log about the spat.
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.
The footage, broadcast by Channel 4 News, appears to show there were few members of the public passing by at the time - apparently contradicting police records.
Mr Mitchell demanded a full probe into the police account of events and insisted the email - sent to MP John Randall - 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."
The police officer reportedly did not disclose his job in the email to Mr Randall, who was Mr Mitchell's deputy in the government whips' office and reportedly suggested he would quit unless his boss left his post.
He described how he had been walking past Downing Street with his nephew when the spat took place.
The email said Mr 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.
Mr 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****** 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, Mr 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."
Former Tory leader Lord Howard told BBC Radio 4's Today: "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."