In yesterday’s Daily Telegraph, Boris Johnson proposed that all the British fighters in Syria should be presumed guilty unless proven innocent. Based on our extensive research and contacts with Western foreign fighters that are currently in Syria and Iraq, this is a dangerous and counterproductive proposal that would increase – rather than diminish – the terrorist threat to our country.

For the last 18 months our research unit at the International Centre for the Study of Radicalisation at King’s College London has been conducting interviews with foreign fighters. We now maintain a database of more than 450 fighters currently in Syria and Iraq.

Their motivations for travelling to Syria are diverse, and it is wrong to think of them as a homogenous group. Some of them will pose a significant national security threat, and some will turn to international terrorism. For them, there must be a strong punitive approach, involving arrest and prosecution.

But tougher laws and blanket punishment shouldn’t be the only approach.

The only authoritative study of the issue, based on nearly one thousand jihadist returnees from previous conflicts showed that one in nine former fighters subsequently became involved in terrorist activity. This does leave a majority who do not wish to become involved with terrorism, for whatever reason. In many cases they are disillusioned, psychologically disturbed, or just tired.

While it is the most ideological, vicious and bloodthirsty fighters who attract the headlines, many have found the reality to be far different from what they were led to believe. “We were pumped up with propaganda,” a foreign fighter, Abu Mohammed (not his real name) told us yesterday.

Abu Mohammed has explained that he, along with scores of other British fighters wants to return to the UK. When he first travelled out there, he said “it was all focused on Assad,” he said. “But now it’s just Muslims fighting Muslims. We didn’t come here for this.”

The blanket approach taken by the government – to threaten all returnees with draconian prison sentences – Abu Mohammed says, makes him feel trapped. “We’re forced to stay and fight, what choice do we have? It’s sad,” he told us.

We have been here before. Following the defeat of the Soviet Union in the 1980s, Arab-Afghan fighters could not return to their home countries. They were stripped of their citizenship and threatened with long prison sentences. Instead, they regrouped in Sudan and formed a Jihadist Internationale, from which al-Qaeda emerged.

It makes perfect sense: isolated, trapped, but searching for meaning and purpose, these men were offered no opportunity to disengage from the path they had chosen. In many cases, they did so on the basis of poor information and the exuberances of youth.

Authorities in the Middle East have learned this lesson. In the late 1990s and early 2000s, Egypt, Saudi Arabia and other countries in the region launched deradicalisation programmes to convince jihadists to disengage. Though controversial – and not without some failures – these programmes have had a profound effect on movements like Gamaa Islamiyyah in Egypt or the Libyan Islamic Fighting Group, both of which disengaged from violence.

It is entirely conceivable, of course, that people like Abu Mohammed could pose a risk if returned to the UK, but these are risks best mitigated by a sophisticated intervention programme which combines deradicalisation along with monitoring and surveillance. Abu Mohammed told us he would be willing to submit to such a scheme, were it available, in order to return to the UK.

Fortunately such a programme already exists and is called the Channel Project. More than 1000 people deemed to be at risk of violent extremism have already been successfully engaged through this programme. Indeed, the Coalition considered Channel to be so successful that in 2010 its funding was increased, while other measures were simultaneously being cut.

There are clearly some terrorists among the cohort with which Abu Mohammed has associated himself. Treating all foreign fighters as terrorists, however, risks becoming a self-fulfilling prophecy. It may sound tough, but it isn’t likely to be effective. Arrests and prosecutions will be needed, but they are just one part of the government’s armoury. It must also offer a way out. This is not about being soft: it’s about being smart.

A programme for returnees would not just focus on de-radicalisation but would also create an opportunity for their continued assessment and monitoring. In prison, by contrast, they are likely to be further radicalised while potentially exposing others to a hardened ideology and worldview.

We asked Abu Mohammed if he would discourage others from coming to Syria at the moment. “Yes,” he said. He told us of another friend who recently quit the fight after he couldn’t accept what he saw out there. These are powerful voices borne of real experience – they need to be heard, not locked away.

Shiraz Maher is Senior Research Fellow at the International Centre for the Study of Radicalisation (ICSR), King’s College London. Peter Neumann is Director of ICSR.

@shirazmaher @PeterRNeumann