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Iraqi national Mohammed Yaseen joined Al Qaeda when he was just 16 years old, and became a member of a cell known for beheading Briton Ken Bigley. He later joined an ISIS unit which engaged in mass executions.

Yaseen was living in a bail hostel in Windsor when he was spotted at the Co-op close to Windsor Castle, after being released from prison last week.

He was serving a six month sentence for arriving on a small boat.

However, he was taken back to prison shortly after the sighting due to breaching good behaviour regulations.

Read more: Knife-wielding small boat migrant who tried to storm Israeli embassy jailed for life

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He gave a false name to border authorities when he entered the country last December, and prosecutor Steven Molloy told Winchester Crown Court at his trial for illegally entering the country that Yaseen had a “lengthy involvement in terrorism and Islamic extremist ideology”.

Despite British courts being told that Yaseen “posed a danger to the whole of the UK,” he is currently still allowed to remain in the country for the time being after reapplying for asylum on human rights grounds.

The Foreign Office has reportedly raised concerns around whether he could face mistreatment if he returned to Iraq due to his past.

The Foreign Office has been contacted for further comment.

Yaseen had previously served a near five-year prison sentence in Germany after being found guilty of being a member of Isis.

Although Germany tried to deport Yaseen back to Iraq, his deportation was halted over concerns for his safety in the country.

His ties to Isis makes him a target for potential execution.

German prosecutors found that he worked as a guard at two Isis mass killings, and controlled crowds at deathly public stoning events.