LONDON (AFP) - A British policeman on Friday admitted he falsely claimed to have witnessed a row over a bicycle that brought down a government minister.

Police constable Keith Wallis, 53, pleaded guilty to misconduct in a public office by pretending he witnessed the argument between former Conservative chief whip Andrew Mitchell and an officer at the gates of Prime Minister David Cameron's Downing Street residence last September.

Mr Mitchell was forced to resign over claims he called officers guarding Downing Street "fucking plebs" because they refused to let him go through the gate with his bike, adding that they should "know your fucking place".

Mitchell admitted he swore but denied using the word "pleb", a derogatory term for the lower social classes. He was forced to resign over the so-called "Plebgate" row a few weeks later.

His allies claim police lied about the incident to discredit PM Cameron's government as it was imposing major cuts to the national police budget.

Wallis, 53, admitted at London's Old Bailey court on Friday that he lied in an e-mail to his local MP that he was present during the row, and admitted arranging for his nephew to support the claim.

The court heard that he has offered to resign from London's Metropolitan Police, where he served as a member of the Diplomatic Protection Group.

Seven other police officers are facing disciplinary proceedings over the scandal.

Wallis was released on bail and is due to be sentenced on Feb 6 pending the outcome of a psychiatric report.

Judge Nigel Sweeney warned him that "all sentencing options remain open to the court".

Mr Mitchell, who is also a former international development minister, welcomed Wallis' guilty plea but said it was "sad and worrying" that a police officer had behaved in this way.

"I am pleased that justice has been done in a criminal court today," he said.

"There remain many questions unanswered, in particular why Pc Wallis wrote this e-mail and who else was involved in this process."

Metropolitan Police chief Bernard Hogan-Howe apologised to Mr Mitchell for Wallis' lie, adding that suggestions that officers had falsified statements were "extremely damaging".

"To lie about witnessing something and provide a false account falls way below the standards that I and Pc Wallis' colleague expect of police officers," police chief Hogan-Howe said.

"His actions have also negatively impacted upon public trust and confidence in the integrity of police officers."

The force said Wallis would face a misconduct hearing once the court case is over.