The increased risk will refocus attention on the decision by Prime Minister David Cameron - backed by MPs in the House of Commons - not to intervene as the Syrian conflict worsened last August.

For the past two years, British extremists have been able to gain access bomb and weapons training as well as further radicalization.

There are fears that British men who have been radicalized in Syria are also being encouraged to return to the UK to carry out attacks here rather than staying to fight. As many as 500 Britons have headed to Syria to fight in the past three years – far higher than the numbers who travelled to Iraq.

The police and security services are understood to be monitoring around half of that number who have returned. Some arrests have been made.

A senior Whitehall source told The Telegraph: “We are seeing a growing threat to the UK from terrorist groups in Syria.

“The threat to the UK comes from a range of countries and groups but Syria [war] is perhaps the biggest challenge right now.”

Confirmation of the escalating concern was made by an additional paragraph to the Government’s assessment on the threat from foreign fighters published on the Security Service’s website.

It says: “The nature of the conflict in Syria and the emergence of the Al Nusrah Front, which has declared its allegiance to Al-Qaeda leader Ayman al-Zawahiri, is leading to the country becoming an increasingly significant potential source of future threats to the UK and UK interests overseas.”

Syria’s proximity to Europe makes it easier and cheaper for would-be extremists to head there and does not appear as “foreign” as the once-Al-Qaeda strongholds of Afghanistan/northern Pakistan and Somalia or Yemen

Monitoring of terrorists in Syria by British intelligence agencies is made more difficult because of the ease with which they are able to move around the country.

Meanwhile, the foreign-sponsored Syrian war was identified as the “most significant development in global terrorism” in the Home Office annual review of its counter-terrorism strategy Contest this week.

NTJ/MB