Some have admitted carrying out armed robberies at home to fund their trips to the Syria and Iraq conflicts while others have convictions for drug dealing and violence, with known links to street gangs.

Their battlefield experience, fanaticism, thirst for violence and links to gangs that would give them easy access to weapons, could enable them to launch terrifying attacks in the UK. Last night a Special Branch source said: “The so-called jihad in Iraq and Syria has been particularly inviting to some young British Muslims who are already involved in violent crime in this country.

“They are attracted by the glamour of fighting for a ruthless organisation like IS and are no strangers to extreme violence and brutality.

“For many it has been like an adventure holiday and some have referred to ‘gangsta jihad’ in their postings on social media. The language they use, referring to their guns as ‘chrome’ and addressing their ‘homies’ back in the UK, is the sort of street slang associated with criminal gangs.”

Terror expert Raffaello Pantucci, who has been studying the background of British men fighting with IS, warned: “The danger is of serious criminals with battlefield experience using this in civilian situations either as terrorist activity or criminal activity.”

There are thought to be 300 to 500 British Islamic fighters in Syria and Iraq, aged 17 to 35. Mr Pantucci went on: “Governments across Europe have noticed this pattern between crime and jihad and are fearful. These individuals will have access to criminal networks should they return and therefore have easier access to weapons.

“They potentially have the mind-set to want to launch punishing attacks within Europe.”

Among the hundreds of Britons who have joined IS is Abdel Majed Abdel Bary, a former rapper from west London who has been pictured holding a severed head in Syria. He says on Facebook how his “lil brother” was in prison, saying “see you in the afterlife inshallah”. He is believed to be referring to Ahmed Kasper Mikhaimar, serving 26 years for murdering a teenager.

Last night Prime Minister David Cameron warned: “This is not a problem miles away from home. It is our concern here and now.

“If we do not act to stem the onslaught of this exceptionally dangerous terrorist movement, it will only grow stronger until it can target us on the streets of Britain.”