Scotland Yard has vowed to "get to the truth" of allegations that a police officer falsely claimed to have witnessed the "plebgate" row that led to Andrew Mitchell being forced to quit as Chief Whip.
The officer is said to have written to his local MP, posing as a member of the public, giving details of the altercation that took place when the senior Tory attempted to cycle out of the main gates in Downing Street.
Number 10 said the claims - which emerged after a member of the diplomatic protection squad was arrested - were "exceptionally serious".
The Metropolitan Police Service said it was conducting a "thorough and well-resourced investigation" that could look at conspiracy. In a statement it said: "The investigation is looking at the alleged unauthorised disclosure of information and the circumstances surrounding a police officer's claim to have witnessed an incident in Downing Street. If any evidence emerges of conspiracy this will form part of the investigation."
The probe comes after CCTV footage of the incident was broadcast for the first time and appeared to contradict parts of a leaked police log about the spat.
Although there is no sound, the MP can be seen with his bicycle talking to three officers by the main gate for around 20 seconds. He then wheels it over to the side gate and exits. The footage, broadcast by Channel 4 News, appears to show there were few members of the public passing by at the time - apparently contradicting police records.
It later emerged the email bearing a close resemblance to the details of the police log was written on September 20 - the day before The Sun first reported claims that Mr Mitchell called officers "plebs". The email was published on the website of Channel 4 News, which drew on its false account of the then Cabinet minister's row with police in last night's report.
The email is littered with grammatical errors, spelling mistakes, inappropriate capitalisation and malapropisms, including a reference to Mr Mitchell's "digesting behaviour". The writer says he recognised Mr Mitchell and "corrected" his nephew, who thought Mr Mitchell was London mayor Boris Johnson.
"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. 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.) One person even remarked as to why was BORIS swearing."