LONDON (CNN) -

UK Prime Minister David Cameron has laid out his battle plan to counter the threat posed by Islamist extremists returning to Britain after fighting with terror groups overseas.

But already there are questions about how effective those measures would be, whether they are legal and whether Cameron, who heads a coalition government, can push them through Parliament.

"Dealing with this terrorist threat is not just about new powers, it is also about how we combat extremism in all its forms," he said as he announced his plans Monday.

His proposals, which come after the UK government raised its terror threat level Friday from "substantial" to "severe," include a radical new measure to ban Britons from coming home once they join jihadi ranks abroad.

"What we need is a targeted, discretionary power to allow us to exclude British nationals from the UK," he said.

Cameron wants to confiscate passports from would-be fighters, before they travel, and ban other suspects from boarding planes.

"Passports are not an automatic right," he said. "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.

"This power will include appropriate safeguards and oversight arrangements."

Work to prepare legislation to allow this will begin immediately, Cameron said.

Civil liberties issue

As well as stopping would-be jihadists, Cameron said Britain needed measures to prevent the return of foreign fighters.

UK authorities estimate that 500 Britons have gone to Syria and Iraq to fight with Islamist groups.

But opposition lawmakers question whether some of the toughest plans are legal.

"We very much want to see the detail of that because some of our MPs say to do that would breach a whole number of legal and international obligations this country has," said Diana Johnson, a Labour lawmaker and shadow Home Office minister for security and crime.

She backs tough action but insists there must be a balance.

"I can categorically say that civil liberties is a big issue in any debate we have about any new powers."

The head of Britain's counter-terrorism efforts, Assistant Commissioner Mark Rowley, said in a statement Tuesday that in response to the raised alert level, more police would be seen on the streets and that there would be an increase in vehicle searches and other security measures.

Secret evidence

Pledges to safeguard citizens' rights ring hollow to British Muslim-convert Cerie Bullivant.

In 2006, he was detained by British security agents under anti-terror laws. He told CNN he was headed to Syria to volunteer helping orphans. The Syrian civil war hadn't started, and ISIS didn't yet exist.

"Suddenly you go on the basis of secret evidence from being an ordinary member of the public to the worst of the worst," said Bullivant. "Terrorism and terrorists are some of the most heinous and horrible of people. I didn't even know what I was accused of."

He believes he came under suspicion because he had unwittingly, he says, become friends with the brother of a jailed terrorist.

Bullivant was never arrested, never convicted of a crime, nor told what evidence security services had against him.

The British government imposed a "control order" on him, banning him from traveling or meeting friends and subjecting him to a curfew. "A couple of times a week the police would come and you can't stop them and they would search your house," he added.

Two years later, a top court exonerated him.