Security forces are preparing for a Paris-style terrorist attack in the UK as counter-terror sources reveal up 450 radicalised Britons have returned from Syria, while Scotland Yard investigates 600 terror cases.

Special forces have been deployed onto Britain’s streets to monitor major stations, shopping districts and other public places amid fears the UK could be the next target.

Charles Farr, director of the Office for Security and Counter-Terrorism, told the Sunday Telegraph that Islamic State was inspiring followers who “couldn’t go” to Syria to stay at home and “undertake attacks”.

He added that “about 750 [British citizens] of interest to the security and intelligence services” had travelled to Iraq and Syria, and that “about 60 per cent have returned.”

He described Islamic State as a form of “ideological grooming” that inspires young people to take up arms against Western democracies, and said those travelling were getting “much younger.”

Speaking about the special forces deployment, one defence source said: “They will be wandering around shopping centres, stations and other crowded potential targets watching what’s going on. They will not be armed.”

Speaking to the BBC’s Andrew Marr Show this morning, Home Secretary Theresa May said that the SAS are already preparing to tackle simultaneous, co-ordinated attacks in the UK.

“Since the Mumbai attack in 2008 we have been building our capability here, building the capability of our police to be able to react to a firearms attack of that sort. We have been increasing the ability of the emergency services to save lives in high-risk conditions. But we will now review that and see if there are any further lessons we need to learn.”

She is due to chair a Cabinet Office Briefing Room meeting – commonly known as COBRA – today to discuss how the British government will deal with the devastating attacks in Paris.

The government will not raise Britain’s terror threat level, however:

“[MI5] has kept the threat level at severe. That still means a terrorist attack is highly likely. “We’ve been at that threat level for over a year now and of course we operate at that threat level and since the attacks [in Paris] that took place on Friday and there’s been an increase in peace presence on the streets and at some events. “Our border force have increased checks they’re making at the border, more screenings of freight vehicles and other vehicles as well. People going through our ports will see a greater police presence.”