A British jihadi who has threatened to ‘bomb the UK’ is today unmasked as a former supermarket security guard – who travelled to wage war in Syria despite being known to police.

Less than two weeks ago, the anonymous extremist known only as ‘Awlaki’ gave an extraordinary interview to the BBC where he boasted about fighting with the terror group Islamic State (IS) and laughed about the beheading of soldiers.

But today The Mail on Sunday can reveal that this fanatical Muslim who claims to ‘hate the UK’ is 27-year-old Omar Hussain, who grew up in High Wycombe, Buckinghamshire.

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Execution witness: Omar Hussain has told friends in the UK that he watched the beheading of James Foley

Astonishingly, he is still in contact with friends in this country and has told them he witnessed the execution of American journalist James Foley.

According to community leaders in High Wycombe, police knew of his extremist views and had arrested him at Heathrow last year on suspicion that he was trying to travel to Syria. Despite this, he managed to leave Britain for the war-torn Middle East earlier this year.

Lord Carlile, the Government’s former adviser on counter-terrorism, last night called for an investigation into how Hussain managed to leave the country.

‘It would be extremely troubling if it emerged that he was allowed to leave the country without a travel document. I would like to know what steps were taken given the knowledge of his intentions,’ he said.

Security job: After leaving school, Hussain worked for this Morrisons in High Wycombe as a security guard

The revelations come after Prime Minister David Cameron claimed the prospect of fighters from IS returning to Britain now poses the ‘greatest and deepest’ security threat in the country’s history. On Friday, the UK terror threat level was raised from ‘substantial’ to ‘severe’, meaning the risk of an attack is ‘highly likely’.

In his BBC2 Newsnight interview this month in which he disguised his true identity, Hussain said: ‘I hate the UK, the only reason why I would intend to return to the UK is when I want to come and plant a bomb somewhere.’

When asked if IS beheaded a number of people after taking towns around Aleppo a fortnight ago, he laughed and said: ‘Yeah, we did. I believe there were three or four guys we beheaded.

‘We then put their heads, as usual, in the middle of the town centre.’

Execution: Less than two weeks ago, Hussain gave an interview to the BBC where he boasted about fighting with ISIS and laughed about the beheading of soldiers. Above, Mr Foley is seen moments before his execution

Former classmates from Cressex comprehensive school in High Wycombe yesterday recalled Hussain as a ‘weird, social outcast’. ‘He would tell girls to dress conservatively and put their hair up, otherwise they would be punished by God,’ said one former pupil.

After leaving school, he worked for Morrisons for a number of years as a security guard. Last night, staff at the High Wycombe store recognised his photo. He had worked there three years ago, one colleague said, but left after vandalising a manager’s car.

Hussain travelled to Syria in January after telling his family he was going to Friday prayers at his local mosque.

Lord Carlile, the Government’s ex-adviser on counter-terrorism, has called for an investigation into how Hussain managed to leave the country

It is thought he went first to France, then overland to Turkey and over the border to Syria. Hussain has since been actively involved in fighting around Aleppo in northern Syria.

The Mail on Sunday has learned that he contacted a friend in High Wycombe in the days after Mr Foley’s execution, saying he witnessed it.

According to an informed source, Thames Valley Police are aware of his claims and know the identity of the individual he contacted. Police refused to answer any questions relating to Hussain, saying they cannot comment on individual cases.

Hussain, who is British-born and of Pakistani heritage, is the youngest of three sons and lived at a semi-detached house in High Wycombe with his mother and brothers.

Community elders said he became radicalised two years ago after he started mixing with a group of hardline Muslims known as salafis at his mosque in the Totteridge area of High Wycombe run by the Wycombe Islamic Society (WISE). Zahid Jawed, a spokesman for WISE, said Hussain did worship at the mosque but denied it had radicalised him.

One of Hussain’s neighbours, who asked not to be named, said that in June last year, he was detained at the airport as he tried to leave for Pakistan as police had suspicions that he might travel on to Syria.

As Hussain was questioned at Heathrow the police came and raided his house, the neighbour added. Hussain was also re-arrested a few weeks later outside his house, but released the next day without charge.