A former MI5 spy has spoken to the BBC about life in the secret service, and it was a pretty damning insight into how the security agency works.

The agent, codenamed Robert Acott, spoke to Newsnight about how he was abandoned by his employer after being diagnosed as suffering from stress, and later, PTSD.

One of the more jaw-dropping anecdotes he shared was the information that MI5 had so few non-white officers, one team leader actually considered "blacking up" agents, which was tried out on a training exercise:

The Islamic targets would either meet at each other's houses or at mosques... you can't get away with going in there. Also they would live in mainly ethnic areas and quite often you would find the only white people in the street were surveillance officers.

When asked how many Muslim surveillance officers were working at MI5 at the time, Acott said he believed there was only one within the entire service, prompting one senior figure to suggest "blacking up" as a means of infiltrating communities.

Acott said he spied for MI5 for 18 years, mostly following Islamic and Irish terror suspects, and was often overwhelmed by how unprepared the service was against attack threats.

Watch the interview in full here.

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