Scotland Yard has vowed to "get to the truth" of allegations that a police officer falsely claimed to have witnessed the "plebgate" row that led to Sutton Coldfield MP Andrew Mitchell being forced to quit as 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 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.
READ MORE: {}
"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.
Most Read
"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."
Tory former shadow home secretary David Davis said his party colleague had been the victim of "a really serious injustice" and should be restored to the ministerial ranks.
"This is a shocking revelation that a police officer seems to have masqueraded as a civilian - more than seems to, I've seen the emails and clearly did so - and acted in a way which effectively completely undermined Mr Mitchell in the eyes of the Prime Minister and the rest of Downing Street right at the point they were making the decision - what do we do, do we support him or not?" he told the BBC Radio 4 Today programme.
"That explains quite a lot of the, frankly, what appeared with hindsight to be daft decisions in terms of the way they handled the whole issue."
He went on: "As a result we have had a serious injustice, a really serious injustice. He should be returned to high office.
"He was a fantastic international development secretary. He probably will not become chief whip again but I think the injustice should be put right."
Don’t miss
He said the content of the email was "virtually identical" to the police log and insisted Scotland Yard still needed to investigate whether the official account was accurate given that it "doesn't appear to fit with the CCTV".
"This is very important because it goes right to the heart of whether that original log was absolutely true. Was the lily gilded? Was it changed in any way?" he said.
"That's vital and I'm afraid Mr Hogan-Howe has to address that issue as part of his investigation."
Mr Davis criticised the Cabinet Secretary and the Met for failing properly to examine the evidence when the allegations first emerged.
"Whether it's the Met investigation or Sir Jeremy Heywood, none of this is exactly Sherlock Holmes," he said.
"All the investigations did not go to the heart. Look at the CCTV, see whether it is consistent with the allegations made, consistent with what's been said. It plainly wasn't.
He said he believed Conservative deputy chief whip John Randall had "no choice" but to pass on the email.
"He probably thought his first duty was to bring it to the attention of Number 10 and let them check," he told the programme.
And he said the Police Federation had "some questions to answer" about its tactics.
"This is an organisation who represent policemen who are supposed to uphold justice and justice has not been at the forefront of their tactics through all of this."