Britain's most senior police officer promised a "ruthless" investigation into an alleged conspiracy against a Cabinet minister "no matter where the truth takes us".
Met Police Commissioner Bernard Hogan-Howe defended his handling of the Andrew Mitchell "plebgate" row after breaking off from his holiday to be briefed on progress.
Mitchell increased pressure on the force on Sunday when he claimed he was the victim of a deliberate attempt to "toxify" the Tories and ruin his career.
The former chief whip was forced to quit his Cabinet post amid a storm of protest - fuelled by the Police Federation - over claims he called officers "plebs" during an altercation in Downing Street.
But last week Scotland Yard opened an investigation into a possible conspiracy against the MP after it emerged an email from a civilian witness backing up the claims was in fact written by another officer.
In a statement, Hogan-Howe said: "The allegations in relation to this case are extremely serious.
"For the avoidance of doubt, I am determined there will be a ruthless search for the truth - no matter where the truth takes us."
He said the force's determination to get to the truth was proved by his decision to devote 30 officers to the task and the arrest of a member of the diplomatic protection squad and a civilian.
"I believe these actions are vital in maintaining public confidence in the police," he said.
Hogan-Howe called for the investigation to be allowed "time and space".
Hogan-Howe has faced criticism of his handling of the case after saying earlier this week that, despite the arrest of the officer, he had seen no evidence that "really affected the original account of the officers at the scene".
He pointed to his choice of Deputy Assistant Commissioner Patricia Gallan, head of Professional Standards, to head the investigation and the fact that it was being supervised by the Independent Police Complaints Commission.
The Commissioner will be grilled on the case when he appears before MPs on the Commons Home Affairs Committee next month.
The Diplomatic Protection Group officer was arrested on December 14 on suspicion of misconduct in public office over the email sent to Tory MP John Randall, the deputy chief whip.
It largely backed up the account of what happened in the official log - subsequently leaked to a national newspaper - that claimed Mitchell called officers "plebs" and "morons" during a foul-mouthed tirade.
The politician admits swearing during the exchange but firmly denies using the word which came to symbolise the controversy which eventually forced him out of office.
CCTV footage from the street, which Mitchell only saw after he was forced to resign and was broadcast by Channel 4 this week, appears to contradict some elements of those accounts such as the presence of public witnesses.
On Thursday, police arrested and questioned a 23-year-old man on suspicion of intentionally encouraging or assisting the commission of an indictable offence on December 14.
Mitchell spoke out in the Sunday Telegraph as he intensified efforts to clear his name and pave the way for a return to the Government's ranks two months after being forced out.
"These awful toxic phrases which were hung round my neck for weeks and weeks in a sustained attempt to toxify the Conservative Party and destroy my career were completely and totally untrue," he said.
He is particularly angry with the Police Federation, which represents rank-and-file officers, whose local branches organised protests by members wearing "PC Pleb" T-shirts and demanded Mr Mitchell's sacking.
Outgoing chairman Paul McKeever has acknowledged concerns it "stoked up" the original incident and successor Steve Williams said one of his first acts on taking over next month would be an independent review of its structures.
Former Met commissioner Sir Paul Stevenson used an article in The Sunday Telegraph to compare elements of Federation tactics with those of "militant trade unionists".
Downing Street meanwhile insisted Prime Minister David Cameron "stood behind" his Cabinet colleague for as long as he could amid criticism from some allies of Mitchell.
Newspapers had quoted members of the Mitchell camp claiming he had been left "swinging in the wind" by the premier who they say failed to act on CCTV evidence that cast doubt on the police account.
Number 10 said Mr Cameron, who has not ruled out a return for Mitchell, had "deep sympathy" with his party colleague.
Shadow policing minister David Hanson said he was concerned that Number 10 and the Cabinet Office had worsened the situation "by treating this simply as a media handling issue".
"The IPCC should be able to follow the evidence wherever it takes them. For instance why was this email from someone purporting to be an eyewitness not checked against the CCTV or given to the police months ago so they could investigate?
"In addition, given the current pressures on the IPCC, they will need to set out clearly their involvement and capacity to pursue this investigation fully to ensure it is independent and wide ranging enough.
"We have raised concerns before about the powers, remit and resources of the IPCC and have called for the establishment of a Police Standards Authority to robustly look into allegations against the police."
Tory former policing minister Nick Herbert called for action to tackle what he described as a "cancer" of corruption within the police.
"The extent of wrongdoing should not be exaggerated, but the cancer must be cut out before it spreads," he said.
"The police do difficult and sometimes dangerous work. They deserve our respect for that and both sides must act to keep it."
Mr Herbert said he did not believe swearing at a police officer was sufficient grounds for a ministerial resignation.
"He apologised and the apology was accepted (by the officer). I don't think that was a resigning matter," he told Channel 4 News.
"The inquiry must take its course but they must get on with it because there is a very great danger, it seems to me, of public trust being undermined by this."
Hogan-Howe resumed his Christmas break this evening, Scotland Yard said.
More:Bernard Hogan-howe Andrew Mitchell Tirade Andrew Mitchell Bike Hogan-howe Plebgate Andrew Mitchell Chief Whip
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