A London policeman has been sentenced to a year in jail for lying about witnessing a government minister swear at police officers.

Keith Wallis claimed he saw Andrew Mitchell insulting police guarding the entrance to Downing Street, the prime minister's residence. Mitchell was forced to resign a senior Conservative Party post soon after the still-disputed September 2012 incident.

Mitchell admitted losing his temper when police refused to let him cycle through the Downing Street gate, but denied claims he called officers "plebs" — a derogatory term for working-class people.

A police investigation found Wallis was not present at the scene, as he had claimed.

Judge Nigel Sweeney told Wallis on Thursday that he was guilty of "devious misconduct which fell far below the standards expected of a police officer."