British Prime Minister David Cameron on Monday announced plans to strip suspected Islamist militants of their passports temporarily, to combat the threat posed by radicalized Britons returning from Syria and Iraq.

The proposals come days after Cameron raised Britain's terrorism alert to its second-highest level, saying Islamic State in Iraq and Syria posed the country's greatest-ever security risk.

An ISIS video released last month, purporting to show a man with a London accent man beheading a U.S. journalist, led to concern that Britons fighting in the region could return and launch attacks on British soil.

"We have all been shocked and sickened by the barbarism we have witnessed in Iraq this summer," Cameron told parliament.

"There are two key areas where we need to strengthen our powers to fill specific gaps in our armoury. These are around preventing suspects from travelling and dealing decisively with those already here who pose a risk."

BRITAIN-SECURITY/THREAT

David Cameron argues ISIS in Iraq poses a direct threat to Britain. (Paul Hackett/Reuters)

Cameron said he would bring in new "specific and targeted legislation" to give the police powers to temporarily seize a suspect's passport at the border to give authorities time to investigate them. Currently only Britain's interior minister has the power to withdraw a passport.

He also said the government would consult on a discretionary power to prevent Britons from returning home if they have pledged allegiance to extremist causes. This would extend existing powers which can only be applied to foreign nationals, naturalized citizens and those with dual nationalities.

The package of measures has been subject to protracted negotiations within the two-party coalition government, with the junior partner Liberal Democrat party wary of bringing in new laws that could limit civil liberties.

Intelligence and security services suspect that around 500 Britons have gone to fight in Syria and potentially Iraq. Cameron has described the extremism posed by the Islamic State group as the biggest security threat of modern times — surpassing that of al-Qaeda — and said it poses a direct threat to Europe.

Legality of proposals questioned

Lawmakers immediately questioned the legality of some of the proposals. The government's former top lawyer, Dominic Grieve, said he was concerned that British nationals could be prevented from returning to the country.

"Not only does it offend principles of international law, it actually would offend basic principles of our own common law as well," Grieve said.

In response Cameron said the current system of prosecuting returning extremists worked, but that it was important to address any weaknesses in it.

The government also said it intended to give police the ability to restrict where people based in Britain who are under investigation can live. Airlines would be legally bound to provide details of passenger lists to authorities, strengthening existing border procedures, he said.

With files from Associated Press