LONDON - The "plebgate" scandal started with an angry exchange over a bicycle in front of Downing Street. The controversy over what a senior politician did or didn't say to officers guarding the prime minister's official residence has now grown into a full-blown crisis that is raising new questions about the ethics of Britain's largest police force.

Scotland Yard's reputation has already been battered over its failure to curb law-breaking journalists and police corruption exposed in the phone-hacking scandal that exploded last year.

The force faces renewed scrutiny after Andrew Mitchell, formerly the Conservative Party's chief whip, said a police report quoting him as abusing officers as "morons" and "plebs" - a term of abuse for working-class people - was based on lies.

"For the next three weeks, these awful phrases were hung round my neck in a concerted attempt to toxify the Conservative Party and destroy my political career," Mitchell wrote in the Sunday Times.

"I never uttered those phrases; they are completely untrue."

Mitchell has long acknowledged losing his temper and swearing as he tried to maneuver his bike into Downing Street on the evening of Sept. 19. He was running late, and officers were refusing to open the main gate, he said. But he has long denied using the term pleb or telling officers to "learn your place," words that he described on Sunday as "a bad caricature of what an ill-mannered 1930s upper-class lout might say."

In Britain, a country very sensitive to issues of social class, the story dominated the headlines for weeks. Some police constables, or PCs, walked around with T-shirts bearing the words "PC Pleb." Political opponents called for Mitchell to lose his job. When an e-mail from what appeared to be an independent witness emerged to corroborate the police account, Mitchell found himself with little choice but to resign in October.

The police account, however, has now been challenged; the independent witness was allegedly a policeman who wasn't even at the scene. Security camera footage taken from Downing Street and broadcast by Britain's Channel 4 didn't seem to line up with the officers' accounts. Two people have been arrested as Scotland Yard has pledged to get to the bottom of what happened.

"The allegations in relation to this case are extremely serious," Scotland Yard chief Bernard Hogan-Howe said in a statement Sunday.