David Cameron has expressed concern that a police officer tried to "blacken the name" of former Cabinet minister Andrew Mitchell amid mounting questions over the initial account of the row that ultimately led to his resignation.
As Scotland Yard vowed to get to the bottom of allegations that an officer falsely claimed to have witnessed Mr Mitchell's row with Downing Street police, the Prime Minister said it was a "very serious issue" which had to be "seriously investigated".
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" when they refused to let him ride his bicycle through the main gates of Downing Street.
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 today 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, on which at least 30 officers are working, 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. It is littered with grammatical errors, spelling mistakes, inappropriate capitalisation and malapropisms, including a reference to Mr Mitchell's "digesting behaviour". The writer says he was walking past Downing Street with his nephew when he recognised Mr Mitchell, who was having a row with the police.
"Imagine to our horror when we heard MR MITCHELL shout very loudly at the police officers guarding YOU (expletive blacked out) PLEBES !!" and "YOU THINK YOU RUN THE (expletive blacked out) COUNTRY" and just continued to shout obscenities at the poor police officers," it says.
"My nephew, as was I, totally taken aback by his, MR MITCHELLS' behaviour and the gutter language he used, especially it appeared directed at the police officers. Now I know that the other people/tourists standing with us were also shocked and some were even, inadvertently filming the incident (it wouldn't surprise me that in this age it'll be on YouTube or other social media website.)" The Metropolitan Police Service said it was conducting an investigation that could look at the possibility of a conspiracy.