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".
"Andrew Mitchell has consistently denied the version of events presented," he added.
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.
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
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.
Four other officers are facing gross misconduct hearings later this year relating to the "improper disclosure of information", the Met said in a statement.
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.