London: A police officer accused of falsely claiming to have witnessed the Plebgate row has pleaded guilty to misconduct in public office.

Constable Keith Wallis, 53, from west London, appeared at the Old Bailey in central London on Friday and admitted committing misconduct in public office between 19 September and 16 December 2012.

Wallis was accused of wrongly claiming in an email to his MP, the Conservative deputy chief whip John Randall, that he witnessed a confrontation between the then chief whip Andrew Mitchell and diplomatic protection officers at the gates of Downing Street on 19 September 2012.

In the email, Wallis purported to be a member of the public and arranged for his nephew to support the claim that he witnessed the incident.

Wallis sat in the dock of the oak-panelled court for a short hearing and spoke only to confirm his name and date of birth, and to plead guilty.

Asked by the court clerk for his plea, Wallis stood and answered: “Guilty, your honour”.

Mitchell, the Tory MP for Sutton Coldfield, welcomed the plea.

He said in a statement that he was pleased to see justice had been done.

“It is very sad and worrying for all of us that a serving police officer should have behaved in this way,” he said. “There remain many questions unanswered, in particular why PC Wallis wrote this email and who else was involved in this process.

“I am looking forward to seeing justice done in the up to 10 other related disciplinary cases involving police officers so that I can focus all my energy on delivering for my constituents and help David Cameron win a Conservative majority at the 2015 election.”

Wallis’s barrister, Patrick Gibbs QC, told Justice Sweeney at the Old Bailey that the officer had admitted the charge in a police interview and pleaded guilty “at the first opportunity” in court.

Wallis has offered his resignation to the Metropolitan police.

Gibbs told the court that to understand the plea it was important to take into account the psychiatric state Wallis “is in now and has been in for many months” as well as his state of mind at the time he sent the email.

But Gibbs said he would not expand further on the basis for the plea.

The row erupted when Mitchell became involved in a heated confrontation with another officer, Toby Rowland, after he was refused permission to cycle through the main gate of Downing Street.

Mitchell admitted swearing but has strenuously denied Rowland’s claim that he used the word “pleb”.

Wallis was the only police officer to be charged over the incident, which sparked a huge row between Scotland Yard and Downing Street and resulted in Mitchell losing his role in David Cameron’s cabinet. The former chief whip has been fighting to clear his name ever since.

Sweeney released Wallis on police bail, saying: “In granting your bail, I am giving you no indication whatsoever as to what the eventual sentence will be.”

In a statement, Scotland Yard said that at the conclusion of the legal proceedings Wallis would face a misconduct hearing in relation to discreditable conduct, honesty and integrity and/or improper disclosure of information.

Commissioner Sir Bernard Hogan-Howe said: “This investigation has been a ruthless search for the truth as at the heart of this are extremely damaging allegations that officers have lied and falsified statements against a cabinet minister.

“The evidence against PC Wallis was such that he has entered a guilty plea. To lie about witnessing something and provide a false account falls way below the standards that I and PC Wallis’s colleagues expect of police officers. His actions have also negatively impacted upon public trust and confidence in the integrity of police officers.

“I would also like to apologise to Mr Mitchell that an MPS officer clearly lied about seeing him behaving in a certain manner. 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.

“As there are still a number of gross misconduct hearings I must take great care not to say or do anything that prejudices the outcome of those very important hearings as these officers have important questions to answer.”