Jeremy Corbyn was monitored by undercover officers for two decades amid fears that he was attempting to undermine democracy, the Telegraph can disclose.

A former special branch officer, who does not wish to be named, said that the Labour leader was monitored because he was “deemed to be a subversive”.

The Special Branch definition someone who is subversive “consists of activities intended to undermine or overthrown Parliamentary democracy by political, industrial or violent means.”

Mr Corbyn would have come under the “undermine” category as if there was evidence anyone was about to overthrow democracy they would have been arrested.

Diane Abbott also had a file open on her during the period in the same “subversive” category, the source confirmed.