Image copyright Getty Images Image caption PC Wallis was not present during the "plebgate" incident in September 2012

A Met policeman has admitted misconduct in public office over the "plebgate" affair and is to offer to resign.

PC Keith Wallis, 53, admitted falsely claiming to have witnessed a row between then-cabinet minister Andrew Mitchell and police in September 2012.

He has been bailed for sentencing on 6 February pending psychiatric reports.

Met Commissioner Sir Bernard Hogan-Howe apologised to Mr Mitchell, who said he was pleased "justice had been done" but that unanswered questions remained.

Prime Minister David Cameron welcomed the guilty plea but said it was "completely unacceptable for a serving police officer to falsify an account of any incident".

Analysis Plebgate has gone on so long that Andrew Mitchell could have cycled around the world and back to Downing Street in the time it has taken to get to the bottom of what happened. And the longer it has gone on, the more it has damaged public confidence in the police, as the Met's commissioner has conceded today. More than half a dozen other Met officers face some form of disciplinary action or rap over the knuckles for their conduct. Three more officers from other forces are also being investigated by the police watchdog. None of them is PC Toby Rowland, the man at the heart of the incident at No 10's gate. He faces no disciplinary or criminal action and stands by his account. What's more, he is suing the MP for libel. Meanwhile, Mr Mitchell says he is suing the Sun newspaper. This is by no means over.

"Andrew Mitchell has consistently denied the version of events presented," he added.

'Clearly lied'

What came to be labelled by the media as "plebgate" occurred when Mr Mitchell was stopped from cycling through Downing Street's main gates by another police officer, Toby Rowland.

Mr Mitchell has admitted swearing during the incident but has denied claims that he swore directly at police or used the word "pleb".

In the aftermath, PC Wallis sent an email to his local MP John Randall, then-Conservative deputy chief whip, wrongly stating he had witnessed the row.

At the Old Bailey on Friday, PC Wallis spoke only to confirm his name and that he understood the charges before he pleaded guilty.

The court was told the diplomatic protection group officer had admitted his guilt in a police interview before pleading guilty at the earliest opportunity.

In a statement, Sir Bernard said the evidence against the officer had led to his guilty plea.

Media playback is unsupported on your device Media caption CCTV of Andrew Mitchell at Downing Street gate with bike

PC Wallis's actions had damaged public trust and confidence in the police and in the integrity of his officers, Sir Bernard added.

"I would also like to apologise to Mr Mitchell that an MPS [Metropolitan Police Service] officer clearly lied about seeing him behaving in a certain manner," the commissioner said.

"I will be writing to him offering to meet and apologise in person.

"I expect my officers to serve the public without fear or favour, where officers break the law they must expect to be held to account and answer for what they have done."

PC Wallis's lawyer said he would offer his resignation from the Met later on Friday.

Further misconduct charges

Media playback is unsupported on your device Media caption Sir Richard Ottaway MP and friend of Mr Mitchell: "It brings the whole Met into disrepute"

Mr Mitchell - Conservative MP for Sutton Coldfield - said in a statement: "I am pleased that justice has been done in a criminal court today.

"It is very sad and worrying for all of us that a serving police officer should have behaved in this way.

"There remain many questions unanswered, in particular why PC Wallis wrote this email and who else was involved in this process."

He added that he was "looking forward to seeing justice done" in the other outstanding related disciplinary cases.

Mr Mitchell is being sued for libel by PC Rowland over comments he made following the incident.

Image copyright PA Image caption Mr Mitchell wanted to cycle out of the Downing Street gates when he clashed with police

Four other officers are facing gross misconduct hearings later this year relating to the "improper disclosure of information", the Met said in a statement.

The story so far... 19 September 2012 Andrew Mitchell and police row after he tries to cycle through Downing Street's main gate

Andrew Mitchell and police row after he tries to cycle through Downing Street's main gate 20 September 2012 The Sun reports Mr Mitchell swore at officers and called them "plebs" who should learn their place

The Sun reports Mr Mitchell swore at officers and called them "plebs" who should learn their place 21 September 2012 Mr Mitchell denies using the word "plebs" but apologises for being disrespectful

Mr Mitchell denies using the word "plebs" but apologises for being disrespectful 19 October 2012 Mr Mitchell resigns as chief whip after weeks of damaging publicity

Mr Mitchell resigns as chief whip after weeks of damaging publicity 18 December 2012 CCTV footage, broadcast on Channel 4 News, casts doubt on the police officers' version of events

CCTV footage, broadcast on Channel 4 News, casts doubt on the police officers' version of events 19 December 2012 Scotland Yard launches an investigation into claims an officer gave false evidence. In the ensuing months eight people are arrested

Scotland Yard launches an investigation into claims an officer gave false evidence. In the ensuing months eight people are arrested 26 November 2013 PC Wallis is charged with misconduct in public office while, separately, five others from the Met face disciplinary action

PC Wallis is charged with misconduct in public office while, separately, five others from the Met face disciplinary action 4 December 2013 The police officer who had been on duty at the Downing St gates, PC Toby Rowland, says he will sue Mr Mitchell over his comments in the wake of the incident

The police officer who had been on duty at the Downing St gates, PC Toby Rowland, says he will sue Mr Mitchell over his comments in the wake of the incident 10 January 2014 PC Wallis admits misconduct and says he will offer his resignation

It added: "Another two officers will be subject to local misconduct in relation to providing inaccurate statements or inappropriate comments and a seventh officer has been recommended for management action in relation to inappropriate comments.

"These processes are currently under way."

Adjourning PC Wallis's case until February, Mr Justice Sweeney said all sentencing options remained open.

The officer remains suspended and - while his lawyer says he will offer his resignation - it is understood that he would need permission from his superiors to do so.

Steve Williams, chairman of the Police Federation, which represents rank-and-file police officers in England and Wales, said: "Despite the high-profile nature of the case and the officer's regrettable actions, one officer admitting one offence should not be allowed to undermine public confidence in the police service."

The "vast majority" of officers conducted themselves "with the utmost integrity and professionalism at all times, often risking their own safety in the interests of the general public and the communities they serve," he added.

Elsewhere, Keith Vaz, who chairs the Home Affairs Select Committee, said the guilty plea appeared to be "a complete vindication" of Mr Mitchell.

"Now is the time to turn the page on this whole unfortunate incident, which took only 45 seconds but has cost the taxpayer hundreds of thousands of pounds, Mr Mitchell his job and damaged the reputation of the police," he said.

For Labour, shadow home secretary Yvette Cooper said policing reform was needed.