A former MI5 spy has broken cover to give us an unprecedented account of his life in service – and describe his anger at the way he was treated.
The agent, codenamed Robert Acott, said he spied for 18 years, mostly following Islamic and Irish terrorist suspects.
He says MI5 pushed him out after he suffered symptoms of stress, later diagnosed as PTSD.
MI5 would not comment on the claims – but security sources said they felt his was only one side of the story. Nick Hopkins reports.
Follow @BBCNewsnight on Twitter
Tweets by BBCNewsnight
Like BBC Newsnight on Facebook