Isis recruits: who's next when they return here?

"We take it very seriously. What took place in Brussels at the Jewish museum shows exactly what can happen," a spokesman said.

"It's not clear how many have already come home or how carefully they are being monitored. We have to work on the suspicion that some may target Jews. The brutality of the Syrian conflict heightens the danger."

Fears grew after it emerged that Reyaad Khan, the Kalashnikov-wielding 20-year-old featured in an Isis recruitment video, had grown up close to the Cardiff homes of three Muslim terrorists jailed in 2012 for plotting to attack British rabbis.

They attended the same mosque, although it is unknown whether at the same time. Their plans were foiled when anti-terror units saw a handwritten list of targets, including the rabbis.

David Cameron has warned that Isis could be plotting to attack targets in this country. It is feared that as many as 250 fighters could have already returned to Britain, some having faked their deaths abroad only to be identified by police here.

Lord Carlile, an expert on terrorism legislation, said all groups are vulnerable.

"There are general threats. When jihadis do return, if there's any reasonable suspicion they have terrorist intentions here then we must have the facilities to prevent them from those acts," he said.

"I'm far from convinced that we do. We need to reintroduce control orders or something like them."

The Jewish peer spent nine years as the independent reviewer of terrorism legislation under the last Labour government and monitored the fight against Islamic extremism.

He also questioned the ongoing suitability of the government's Prevent counter-terror strategy, which is meant to stop the radicalisation of Muslims.