Though the masked militant who was shown directing the killing of five men in an IS propaganda video has not been officially identified, media including the BBC and the Daily Telegraph have named him as Siddhartha Dhar, a Londoner who once sold inflatable bouncy castle toys.

Dhar, 32, left Britain for Syria in September 2014 while on police bail after his arrest on suspicion of belonging to a banned group and encouraging terrorism.

"The system has failed because it allowed him to abscond to Syria," Andy Burnham, the home affairs spokesman for the opposition Labour Party, said in parliament on Tuesday.

"Even if the correct procedures were followed, I have evidence that they were far too weak," he said.

Britain's independent reviewer of terrorism laws said the main failing in the Dhar case was not asking him to hand over his travel papers immediately.

Burnham cited a police letter to Dhar dated Nov. 7, 2014 setting out bail conditions, including surrendering his passport, that was sent six weeks after the militant had left Britain. In the letter, Dhar, who was said to have fled with his pregnant wife and four children, was urged to contact police.

Dhar turned up in Syria two weeks after the letter was sent brandishing a gun and a baby and taunting Britain for its "shoddy" security that allowed him to "breeze" into Syria.

Concerns over the ease with which a man suspected of terrorism offences could flee Britain add to worries about the unhindered travel within Europe of the militants behind the Nov. 13 Paris attacks which killed 130 people.

Abdelhamid Abaaoud, suspected mastermind behind the Paris attacks, had also mocked European frontier controls and boasted how easy it was for him to move between Syria to his Belgian homeland and the rest of Europe. 

BORDERLESS EUROPE?

Since the Paris attacks, the European Union has stepped up checks on its citizens travelling abroad and is seeking more data on passenger travel.

Still, the flow of foreign fighters to Syria has so far shown no sign of decreasing and the United States has called for better cross-Atlantic information sharing to prevent attacks.

The identity of the militant in the Islamic State film has stoked intense speculation because it revived memories of "Jihadi John", another British militant who was shown in IS videos beheading U.S. and British hostages.

Jihadi John, unmasked as Londoner Mohammed Emwazi, was killed by a joint U.S.-British missile strike in November.

British Home Secretary (interior minister) Theresa May, who refused to comment on speculation about the identity of the English-accented militant in the Islamic State video, said Britain had tightened exit controls.

"They are now taking place at the various ports of exit and in a variety of ways, according to how the information about someone's exit is being held," she said.

She added that the decision on whether to place a suspect on police bail was an operational matter.

Syria has become the pre-eminent global incubator for a new generation of militants after Islamist groups more than doubled their recruitment of foreign fighters to as many as 31,000 over the past 18 months, former senior British intelligence official Richard Barrett said last month.