At the time of his release, the then home secretary David Blunkett said: “No-one who is returned...will actually be a threat to the security of the British people.”

Earlier this week Isil named him as the man who had blown himself up in the car bombing at the Iraqi army base, and released a picture of him.

Al-Harith’s wife Shukee Begum travelled to Syria with their five children to try to persuade her husband to return to the UK, but failed and was taken hostage before eventually managing to escape.

Al-Harith, the son of Jamaican immigrants, converted to Islam in the 1990s and worked as a web designer before he travelled to the Pakistani city of Quetta in 2001 for what he claimed was a religious holiday.

He has insisted he tried to enter Iran when the US invaded neighbouring Afghanistan, but was captured and imprisoned by the Taliban on suspicion of being a UK spy.

When US special forces found him in a Taliban jail, they assessed him as a “high threat to the US” who was “probably involved in a former terrorist attack against the US”.

Al-Harith’s prisoner file from Guantanamo Bay, published online by WikiLeaks, refers to him travelling to Sudan in 1992 with “Abu Bakr, a well-known al-Qaeda operative”.