Scotland Yard has widened its investigation into the Andrew Mitchell "plebgate" affair amid signs of strain between senior Conservatives and the police over the treatment of the former chief whip.
Some 30 officers are working on the Metropolitan Police inquiry amid claims that officers conspired to fabricate evidence against Mr Mitchell that ultimately led to his resignation.
David Cameron expressed concern on Wednesday that an officer tried to "blacken the name" of Mr Mitchell amid mounting questions over the initial account of his row with Downing Street police who refused to let him ride his bicycle through the main gates of Downing Street.
The officer is said to have written an email to his local MP, Conservative John Randall, posing as a member of the public and accusing Mr Mitchell of calling police "plebs". The account contained in the email, written the day before The Sun first broke news of the row on September 21, was very similar to that in the police log, which was later leaked to the Daily Telegraph.
Supporters of Mr Mitchell, who finally resigned in late October after a month of highly damaging headlines and criticism made his position as chief whip untenable, sought to start rehabilitating his battered reputation and raised the prospect of his eventual return to the Cabinet.
In the Commons, Mr Cameron said: "A police officer posing as a member of the public and sending an email potentially to blacken the name of a Cabinet minister is a very serious issue and does need to be seriously investigated. The Metropolitan Police Service is conducting a thorough and well-resourced investigation to get to the truth of this matter as quickly as possible."
The Met's investigation is being supervised by the Independent Police Complaints Commission (IPCC).
The email to Mr Randall, which was passed to Downing Street on September 25, was published by Channel 4 News on Wednesday night. It is littered with grammatical errors, spelling mistakes, inappropriate capitalisation and malapropisms, including a reference to Mr Mitchell's "digesting behaviour".
The Metropolitan Police Service said it was conducting an investigation that could look at the possibility of a conspiracy. In a statement, it said: "The allegation that a serving police officer fabricated evidence is extremely serious. It goes to the very heart of the public's trust in the police service."
An officer was arrested on Saturday on suspicion of misconduct in public office.