Security forces have estimated that up to 100 Britons have been killed fighting with terrorist groups such as Islamic State (Isis) and Jubhat al-Nusra whilst an estimated 50 "high-risk" fighters are back on UK streets. British security services say they are keeping a close eye on the returning jihadis in order to prevent a Paris-style terror attack on British streets.

Only last week British foreign secretary Philip Hammond announced that 600 Brits had been intercepted attempting to enter Syria via Turkey since 2012. Turkish authorities told the Conservative MP that some of the Brits, alongside Germans, French and Russians, were trying to head to the Isis (Daesh) stronghold of Raqqa in the war-torn nation.

The MP for Runnymede and Weybridge added that 800 Brits had entered Syria, for a range of reasons including joining Islamic State and other militant groups and humanitarian missions, with 400 still living there. Within that number was Mohammed Emwazi, the British killer known as "Jihadi John", who died in an airstrike in November.

In December 2015, the Pentagon said that Siful Haque Sujan , a senior IS hacker from Cardiff, was killed in a drone strike on the self-declared capital of the caliphate. Cardiff-born Reyaad Khan, 21, and Ruhul Amin, 26, from Aberdeen, have also been killed in drone attacks.

Siful Haque Sujan
Siful Haque Sujan was killed by a US air strike on the Isis stronghold of Raqqa in December 2015 Linkedin

It is also estimated that 100 British children, including newborn babies, are also now living under the rule of IS, say intelligence sources. In December 2015 reported militant Siddartha Dhar, also known as Abu Rumaysah, appeared in a propaganda video with a British child.

The child declared that he wanted to kill the Kuffar (non-Muslims) and now Abu Rumaysah is subject to scrutiny at the hands of MI5. It has been reported that 60 girls and women had left for Syria in the past year alone.

"There's been a lot of British people killed out there — up to 100 now," a security source said to The Sunday Times. "And there's every chance that that number will continue to grow.

"We have been warning about the danger of the Syrian battlefield for several years and saying that the reality of the danger on the ground is not the glamorous picture portrayed in the Isis propaganda videos. If you go out, there's a good chance you will be killed."

The security source added that increased coalition airstrikes or brutal internal disputes between terrorist groups have made joining IS or the al-Qaeda linked Jubhat al-Nusra a more deadly proposition than ever.

The 50 "high-risk" fighters are said to be under close surveillance by MI5 with phone taps and travel restrictions.