Britain should resist a rush to overhaul its fundamental legal principles in the face of an "unproven threat" from homegrown militants fighting in Syria and Iraq, the former global counter-terrorism director of MI6 has said.

In an interview with the Guardian, Richard Barrett criticised government plans for new laws to tackle British extremists and warned against Boris Johnson's suggestion that Britons who travel to Iraq or Syria should be presumed guilty of involvement in terrorism unless they can prove their innocence.

"This fundamental tenet of British justice should not be changed even in a minor way for this unproven threat – and it is an unproven threat at the moment," Barrett said.

In a newspaper column described as "draconian" by the former attorney general, Johnson called for British jihadists to lose their citizenship, proposed the return of control orders and urged David Cameron to intervene against Islamic State (Isis) militarily.

The London mayor wrote that Isis, whom he described as "wackos", now controls an area the size of Britain and that the government had to be far more effective at preventing Britons from travelling to Syria or Iraq to join them. "The law needs a swift and minor change so that there is a 'rebuttable presumption' that all those visiting war areas without notifying the authorities have done so for a terrorist purpose," he wrote.

But Barrett, formerly a counter-terrorism chief at both MI5 and MI6, said the government needed to better understand the domestic threat posed by Isis before introducing new laws. "I don't think we should change the laws without a very much more thorough assessment and understanding of the threat," he said.

"Sure, there's a problem with people who go to Syria and they may have broken the law if they joined organisations like Islamic State and al-Nusra Front, but there should be some sort of effort to prove that, rather than assume they've done so."

The home secretary, Theresa May, said last week that banning orders for extremist groups would be considered again – even if they "fall short of the legal threshold for terrorist proscription" – alongside powers to stop radical preachers. However, Barrett said tighter rules could curb the free speech of groups whose sermons do not "obviously and directly incite to violence". He said: "The banning of groups that fall short of violent extremism but appear to promote it should follow a clearer analysis of what makes people leave the UK to join a group like the self-described Islamic State. Maybe it will, but if so, I have not seen the analysis and wonder on what it will be based."

Dominic Grieve, the Conservative former attorney general, also suggested it was unwise to propose major changes to the law on the basis of a single horrific incident such as the killing of the American journalist James Foley.

Grieve dismissed Johnson's proposal as "draconian" because it would throw out the ordinary principles of common law and potentially lead to the prosecution of people who legitimately travelled to war-torn Middle East states.

He told BBC Radio 4's World at One: "Boris Johnson is suggesting that effectively there should be rebuttable presumption, so that the moment it was established by a prosecutor that you had gone to a country like Syria the burden would be on you to show that you were acting innocently."

The former minister also dismissed the call from Johnson and Conservative backbencher David Davis for British jihadists to be stripped of their citizenship even if they were born in the UK and hold no alternative citizenship. Under current rules the Home Office can only do this to naturalised Britons, or those with dual citizenship. The proposal was "entirely contrary to a United Nations convention of which we are signatories", Grieve said. "If we are about to rip up a UN convention, we need to think through the consequences."

The deputy prime minister, Nick Clegg, added his concerns to calls for new laws, describing the tougher control orders introduced by the Labour government as "fundamentally flawed" and stressing that Britain already has a number of measures to tackle potential terrorists. "I sometimes wish it was as simple as Boris Johnson implies: all we need to do is pass a law and everything will be well," he said during a visit to India.

Johnson has a track record of using his Daily Telegraph platform to float alternatives to government policy and No 10 sources were phlegmatic about his intervention, pointing out that the prime minister and Home Office had already set out what the government is doing to counter the Isis threat internationally and domestically in lengthy newspaper articles this summer.

But the fact that figures such as Johnson and Davis are so keen to float alternative measures, even ones that are legally questionable, indicates that Cameron and May are failing to persuade colleagues that they are doing everything necessary.

The net appeared to be closing on the British man dubbed "Jihadi John" at the weekend when the British ambassador to the US, Sir Peter Westmacott, said voice recognition technology had been used to pin down the identity of the man.

But the MI5-led investigation has not so far deterred other British militants from boasting about their actions in Syria. Nasser Muthana, 20, a former medical student from Cardiff, on Monday bragged on Twitter about forcing members of the Yazidi community to convert to Islam and undergo a form of spiritual healing – days after claiming there were "hundreds of Yazidi slave women" in Syria.

Muthana tweeted: "Converting Yezidis even the jins of them lol, alhamdulillah ruqyah today Yezidi jin accepted islam and knows conditions of la ilaha ilallah".

Earlier on Sunday, the British jihadist took to Twitter to tell a US TV network it should be "working to save stevie boy" – the second hostage held in the Isis video of James Foley's murder – instead of trying to identify Foley's killer.

Muthana, who appeared in a widely circulated recruitment video for Isis, travelled to Syria with his brother, Aseel, 17. Previously, he has boasted about acquiring bomb-making skills and posted pictures apparently showing the charred remains of Syrian army soldiers.

In recent days, the Cardiff-born militant has bragged about the number of Yazidi captives, tweeting: "We have hundreds of Yazidi slave women now in Syria, how about that for news!" Responding to a tide of online criticism about his slave comments, Muthana wrote: "When I spoke about slave everyone jumped on me muslims and non muslims alike … so I stayed quiet and will stay quiet but everyone will soon find out when I get my own concubines lool, slave markets are on full blast."

The Muthana brothers, who grew up in Cardiff after their father moved there from Yemen as a teenager, are among an estimated 500 young men from Britain who have flown to Syria to join the rebels.

One of Muthana's associates, using the name Abu Dhar Alhumajir, wrote in another message: "The price of one slave girl is about $1,500-2,000. I think female captives/slaves would entice a lot of people."

On Sunday, Alhumajir – thought to be a Briton of Somali origin – tried to recruit two other Twitter users to fight alongside Isis in Syria. He told one Twitter user that women were joining militants every day, "the problem is you not trying hard enough full stop". He added: "U should feel ashamed that sister are making hijra while u complain about how hard it is".