The number of Muslim extremist citizens returning from fighting in the Syrian civil war to the United Kingdom is much greater than previously estimated, and British security forces are stepping up their intelligence operations in light of this revelation, it was reported on Sunday.

About 250 British citizens have recently returned home from attending training camps or participating in operations with Muslim jihadi elements of the Syrian opposition, a figure five times the previous estimate, the Sunday Times reported.

Many of these fighters have participated in combat and have training in munitions or other aspects of terror activity, and some are willing to carry out actions in the UK, according to security officials cited in the report.

Most of the jihadis arrive to Syria via Turkeyand were able to covertly join one of the several UK-organized aid caravans, which ostensibly bring humanitarian help to Syrian refugees. Once in Syria they may stay for as little as a few weeks or as long as many months.

The UK jihadis may then receive, in addition to regular training and the opportunity to fight against the Assad government, special instructions on how to carry out effective terror attacks on British soil, the report noted, and added that some of the UK citizens may in fact be teaching jihadi extremists from a multitude of countries about conditions in England.

“There are a few hundred people going out there. They may be injured or killed, but our biggest worry is when they return they are radicalized, they may be militarized, they may have a network of people that train them to use weapons,” London police chief Sir Bernard Hogan-Howe told the paper.

Reportedly, last year MI5 agents thwarted a cell of returnees who were planning a large-scale mass shooting and bombing attack, similar to the Mumbai 2008 multi-day terror attack perpetrated by Pakistani extremists.

A government official stressed to the paper that not all the returnees are considered a threat. “Many will have returned and want nothing more to do with it. Others may be arranging training or simply moving money,” he said.

The report noted that UK authorities are investigating nine charities which purport to raise money for humanitarian assistance in Syria, but are thought to funnel some of their funds to the Syrian opposition.

This week, a video surfaced online purporting to show UK citizen Abdul Waheed Majeed, 41, minutes before he drove an explosive-laden truck earlier this month into a jail in Aleppo, Syria, which allowed dozens of rebel fighters to escape. Majeed is thought to be the first known British suicide bomber, the report noted, and is one of around 20 UK citizens believed to have perished in Syria.