A British Islamic State fighter believed to have carried out a suicide bombing in Iraq was a former Guantanamo Bay detainee who won a £1 million payout from the Government.

The bomber, named by the group as Abu Zakariya al-Britani, is said to have detonated an explosives-filled vehicle in a village south of Mosul.

He has been identified as Muslim convert Ronald Fiddler, who was also known as Jamal al Harith or Jamal Udeen.

Suspected of terrorism by the Americans, he was freed from the US detention centre in 2004 after lobbying by the British government.

Fiddler, 50, reportedly alleged torture and received £1 million in compensation from the Government after his release, along with four other Britons.

He is believed to have travelled to join IS fighters in Syria in early 2014. His wife, Shukee Begum, reportedly took her five children to Syria to try to "speak some sense" into him later that year.

A former senior security official admitted UK authorities failed to keep track of him.

Arthur Snell, former head of the Prevent programme at the Foreign Office, told BBC Radio 4's Today programme: "It's obvious that collectively, the authorities - and obviously I have some personal responsibility there - we failed to be aware of what Fiddler was up to."

Lord Carlile, the former terror laws watchdog, said compensation was paid to avoid disclosing sensitive documents in a court battle.

He told Today: "The government was quite right not to want to disclose those files but plainly there is an anomaly between the United Kingdom, where the threat of disclosure of such files led to the payment of compensation, and the United States, where this man brought a legal action which failed.

"It should never have been paid on the merits. There was absolutely no merit in paying him a penny because plainly he was a terrorist and he was a potentially dangerous terrorist.

"The issue was the legal disclosure rules. If someone brings a civil action for damages they are entitled to disclosure of material, some of which may be national security material."

At the home of Fiddler's sister Maxine, in Withington, Manchester, the Press Association was told she did not want to speak.

A woman at the address of his father, Lloyd Fiddler, in Moss Side, also declined to comment and slammed the door.

His brother, Leon Jameson, 53, identified him as the man equipped with explosives in an IS propaganda video.

Mr Jameson told The Times his brother had "wasted his life" and added: "It is him, I can tell by his smile."

Fiddler was released in March 2004 along with four other British detainees from the internment camp in Cuba where they had been held for up to two years over alleged links to al Qaida and the Taliban.

The group included Shafiq Rasul, 26, Asif Iqbal, 22, and Ruhal Ahmed, 22, all from Tipton in the West Midlands, and Tarek Dergoul, 26, from east London.

Former prime minister Tony Blair came under fire for the reported lobbying by his government for the Guantanamo Bay prisoner's release.

The Daily Mail quoted Tory MP Tim Loughton as saying: "So much for Tony Blair's assurances that this extremist did not pose a security threat. He clearly was a risk to Britain and our security all along."

Around 850 people of national security concern have travelled to join the conflict, according to figures published by the Government last year. Just under half have returned to the UK and about 15% are dead.