Police constable Keith Wallis, 53, pleaded guilty to misconduct in a public office by pretending he witnessed the brief argument between the then government chief whip Andrew Mitchell and an officer at the gates of Prime Minister David Cameron's Downing Street residence last September.
Mitchell was forced to resign over claims that he used abusive words for officers guarding Downing Street because they refused to let him go through the main gate with his bike.
Mitchell admitted he swore but denied using the word "pleb", a derogatory term for the lower social classes. He resigned over the so-called "Plebgate" row a few weeks later.
Wallis, 53, admitted at London's Old Bailey court today that he lied in an email to his local lawmaker that he was present during the row, and admitted arranging for his nephew to support the claim.
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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.
Six other police officers are facing internal disciplinary proceedings over the scandal, the force said.
Wallis was released on bail and is due to be sentenced on February 6 pending the outcome of a psychiatric report.
Mitchell, who is also a former international development minister, welcomed Wallis's guilty plea but said it was "sad and worrying" that a police officer had behaved in this way.