A 23-year-old man has been arrested over the 'plebgate' affair involving Andrew Mitchell.
The arrest comes amid a growing concern in Downing Street at the actions of police involved at the scene.
The man was been taken into custody in relation to "intentionally encouraging or assisting the commission of an indictable offence".
Scotland Yard said the man is "not a police officer or a member of police staff".
Thirty officers have now been put on the case. It follows an arrest of a serving police officer at the weekend on suspicion of misconduct in public office, following the leaking of the police log to a national newspaper.
The plebgate scandal has leapt back to the forefront of British politics after CCTV footage emerged earlier this week casting doubt on the account provided by Met officers on duty at the gated entrance to Downing Street.
Allegations that a police officer could even have posed as a member of the public in a bid to "blacken the name of a Cabinet minister" were described by prime minister David Cameron as a "very serious issue" in the Commons yesterday.
"The Metropolitan Police Service is conducting a thorough and well-resourced investigation to get to the truth of this matter as quickly as possible," Cameron said.
"The Independent Police Complaints Commission will be supervising the investigation, and I think we should allow it to get to the truth."