London’s Chief of Police has today issued a warning that around 200 “militarised” ISIS fighters are already back on the streets of London.

The figures, quoted in the Evening Standard, have intensified the debate raging between Britain’s counter-terror heavyweights over how to deal with this escalating problem.

Sir Bernard Hogan-Howe, the Metropolitan Police Commissioner revealed in his interview on LBC radio this morning: “We think at least 500 to 600 went. Some have come back. We’ve got to keep an eye on them. They risk being militarised and they are in a network of terrorists. It’s a worry because we think two-thirds to three-quarters of them come from the London area, so that’s quite a large number.”

Five to six hundred is the highest number yet quoted by the police, but falls significantly short of the 1,500 fighters claimed by Labour’s Khalid Mahmood MP, who called the official figure “nonsense” last week.

Hogan-Howe made a strident call for action over the security hazard posed by returning fighters by echoing the words of London’s Mayor Boris Johnson. He said there should be a presumption of guilt over British citizens who travel to Iraq and Syria without good reason to help police deal with the situation. While admitting a change in law should be left to the politicians, he said: “Anything that either stops them going or frankly stops them coming back is a good idea. If in law it works, we should do that.

“It’s a privilege to have a passport, be a citizen of this country, and if you are going to start fighting in another country on behalf of another state or against another state, it seems to me you’ve made a choice about where you want to be.”

This is just the latest blow in the war of words between senior figures over how the law should be used, or changed to deal with the phenomenon of British citizens going abroad to fight. Richard Barrett, the former director of MI5, MI6, counter-terrorism adviser to the Home Office, and one-time forceful critic of British fighters abroad made an unlikely intervention yesterday in the Guardian.

Despite recently authoring a strongly worded report on the danger of foreign fighters in Syria, which he calls “an incubator for a new generation of terrorists”, and an environment where more radical groups flourish at the expense of moderates, Barrett has apparently suffered a change of heart. Taking a position that is very clearly at odds with the London Mayor, the Police Commissioner and his own former views he said “I don’t think we should change the laws without a very much more thorough assessment and understanding of the threat”.

Barrett had previously warned that resources available to police and security services to track ISIS fighters in the West were “insufficient to monitor more than a handful of returnees”.