Prime Minister David Cameron on Monday proposed new laws that would give police the power to seize the passports of Britons suspected of having traveled abroad to fight with terrorist groups.

Speaking to Parliament, Cameron said his government is also working on plans to block such suspected British jihadi fighters from re-entering the U.K. The power to monitor such suspects who are already in Britain would also be strengthened.

“What we need is a targeted power to exclude British nationals from the United Kingdom,” the prime minister told lawmakers in London today. “We need to address any potential gap in our armory to keep our country safe.”

The plans to widen Britain's anti-terror laws, which are likely to be approved by parliament, are aimed at preventing attacks by Islamist militants returning from terror training in trouble spots in the Middle East.

Like other Western countries such as the U.S., France and Germany, Britain is worried that citizens who travel abroad to join terror groups could threaten their home country when they return.

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-Qaida — and said it poses a direct threat to Europe.

Britain's Home Secretary already has the authority to withhold passports in some cases, but Cameron said more is needed to ensure police at border crossings could act in time when they spot a suspect.

"We will introduce specific and targeted legislation to fill this gap by providing the police with a temporary power to seize a passport at the border, during which time they will be able to investigate the individual concerned," he said.

Authorities on Friday raised Britain's terror threat alert from "substantial" to "severe," the second-highest level, in response to the crises in Iraq and Syria and concerns that terrorist groups could target Europe. The alert means that an attack on Britain is "highly likely" — though the government did not provide information to suggest an attack was imminent.

On Aug. 20, Islamic State released a video of a masked militant, apparently with a British accent, beheading the U.S. journalist James Foley. Security services on both sides of the Atlantic are working to identify the man and Cameron has asked officials what else they could do to prevent young Muslims becoming radicalized and traveling to conflict zones.

While the government has the power to revoke a passport, police are currently unable to exercise it when they encounter someone at the border whom they suspect of planning to travel to a war zone, the prime minister said. Police will get the power to confiscate travel documents at the border while investigations are carried out, Cameron said.

Cameron also said guidelines requiring airlines to share flight data with security services will be formalized, and those who fail to comply won’t be allowed to land in Britain.