Banning British Muslims fighting in Syria and Iraq from returning home would be a "disaster", according to a leading expert on the root causes of terrorism.

Professor Kamaldeep Bhui, from Queen Mary, University of London, said he would be happy to help returning jihadis.

Evidence suggested they were likely to have been living out a romantic fantasy, only to have their illusions shattered when they reached the war zone.

As many as 500 British citizens are thought to be fighting in Syria and northern Iraq. They were shockingly personified by "Jihadi John", the London-accented member of Islamic State (IS) who has appeared in videos presiding over the beheading of western hostages.

Widely-trailed Government plans to bar them from re-entering the UK have been put on hold amid legal uncertainty and political objections.

Prof Bhui was asked what he thought about a ban at a news briefing to discuss recently-published findings from a study of extremist sympathies among Pakistani and Bangladeshi communities in the UK.

He said: "My personal view... I think it would be a disaster, because I think you're criminalising them. You're disowning British Muslims and saying it's somebody else's problem.

"You'll never get to understand the problem. Some of these kids - 15,18 - they're young and probably inexperienced."

A former counter-terrorism chief at MI5 and MI6, Richard Barrett, shares his view. He has said returning fighters could prove an invaluable asset by persuading potential jihadis not to sign up with IS.

Prof Bhui said he believed many of those travelling to Syria and Iraq were misguidedly pursuing a romantic dream.

"I think it's a fantastic, romantic idea," he said. "The evidence from those wanting to return is that they're saying they want to come back because it's not what they want it to be."

The professor led a study of 608 ordinary members of a Pakistani and Bangladeshi community at undisclosed locations in the UK who were questioned about their sympathies with Islamic extremism and terrorism.

He found that 2.5% had some sympathies and up to 1.5% sympathised with the most extreme acts, such as suicide bombing.

Most of those expressing sympathy with terrorism or radical views were young, British-born, well-educated and from well-off families. They were also likely to be affected by depression, socially isolated, and feeling "disconnected" from their cultural heritage.

Significantly, women were just as likely to be sympathetic as men - even in relation to terrorism.

In contrast, individuals who opposed Islamic extremism were more likely to be migrants, to have a wide social network, and to be from more deprived backgrounds.

"They were the ones who had lower social capital, lower satisfaction with the area they lived in, a lack of trust with neighbours, and a lack of safety in their environment," said Prof Bhui.

"We expected that to be more common among those who were sympathetic, but we found the opposite."

Possibly the newly-arrived migrants struggling to make a life for themselves in the UK were just "too busy" to let themselves be radicalised, he said.

The research also found that contact with established orthodox religion had a protective effect. Radicalisation often began with a "non-orthodox" religious influence.

"They think they're doing the right thing by connecting with a more orthodox form of Islam than their parents, for example," said the professor.

Asked if he would be willing to help jihadis returning to the UK, he said: "I'm happy to work with them when they get back."