London: Nick Clegg has said it is not obvious how the UK can withdraw passports from British jihadis and remain inside international law.

The deputy prime minister indicated on Tuesday that the idea was unlikely to work, saying the UK would not do anything illegal.

The government has been examining the possibility of withdrawing citizenship from suspected jihadis returning from Iraq and Syria. However, David Cameron acknowledged on Monday there were legal difficulties and omitted it from his package of measures to deal with the problem.

On Tuesday morning, David Anderson, the independent reviewer of terrorism, said there were significant difficulties with giving police the powers to withdraw passports from UK citizens, effectively leaving them stateless.

Speaking on BBC Radio 4’s Today programme, Clegg defended measures unveiled by Cameron to tackle Britain’s terror threat, which has been upgraded to severe.

“What the prime minister equally said is that we need to have a discussion and examine how we can and whether we can do more to stop people returning in the first place,” Clegg said.

“At the moment, it is not obvious what one can do that is consistent with our legal obligations. As the prime minister quite rightly said, and I’ve also stressed, we’re not going to do anything which clashes with both our international and domestic legal principles and obligations.

“The others measures taking people’s passports away temporarily at the British borders if they want to leave, drafting legislation so that we can protect existing powers on passport confiscation under so-called royal prerogative those are significant, specific and targeted measures which he announced yesterday to fill the gaps that clearly do exist.”

The Liberal Democrat leader sounded a note of caution about forcing terror suspects to move to different parts of the country.

“Moving people against their will from one part of the country to another when you can’t prosecute in court is of course a big step. And that’s why we’re looking within government to see whether you can maximise and build upon existing powers in Tpims [terrorism prevention and investigation measures] which is all about excluding people from particular areas, but there’s no debate or dispute about the central observation that David Anderson has made, which is that Tpims, to be effective, we need in one way or another, to do more to disrupt the patterns of association that people subject to Tpims have with other individuals.”

Liberal Democrat sources said they had not definitely signed up to the plans for relocation, but said they felt a duty to look at the proposal after it had been recently recommended by Anderson.

After Clegg’s interview, Yvette Cooper, the shadow home secretary, said she was “utterly confused” about the government’s security policy. She questioned why Cameron had made “big claims” about measures to tackle terrorism last week when the details were not worked out. Government sources had also indicated on Friday that they would look into making it easier to revoke passports from returning suspect jihadis.

Boris Johnson, the London mayor, compounded the government’s difficulties by repeating his call for suspected British jihadis returning from Syria or Iraq to be stripped of citizenship.

He told LBC radio: “If people are going to go off to the Islamic Republic of Syria and Iraq or whatever it’s called, and they are going to announce their allegiance to that state and they’re going to engage in jihad or whatever they want to call it, then I think they have effectively forfeited their right to UK citizenship and I have no problem at all with them being stripped of it upon their return and I think that should be made as simple as possible.

“The thing I called for which nobody seems to have gone for yet but I think is perfectly sensible is, if you are going to go to that war zone, all you have to do is identify yourself to the authorities, say ‘I’m going for humanitarian purposes’ or whatever, explain what you’re going to do, people will say ‘all right then, we can’t stop you’, then when you come back people will accept that. But if you go and come back without giving any good reason at all, I think people are entitled to be suspicious.”

On Monday, Cameron told parliament the UK needed “clear discretionary power to allow us to exclude British nationals from the UK”. However, the measures only included a power for the police to revoke a passport temporarily for 30 days, which will be subject to judicial review.

Acknowledging the legal difficulties in preventing British citizens returning to the UK, admitting that it might render them stateless, the prime minister said measures were still needed to prevent British jihadis returning.

He told MPs: “It is abhorrent that people who declare their allegiance elsewhere can return to the United Kingdom and pose a threat to our national security. We are clear in principle that what we need is a targeted, discretionary power to allow us to exclude British nationals from the UK.”

The prime minister’s comments did not match the rhetoric of Friday when he used a Downing Street press conference to warn of the dangers of the “generational struggle” posed by the emergence of Islamic State of Iraq and the Levant (Isil).

The difficulties facing Cameron were underlined by the former Conservative attorney general Dominic Grieve, who warned that removing passports from UK-born citizens returning home would breach international law and UK common law.

Grieve said “even taking such powers on a temporary basis is likely to be a non-starter”.

It is thought Grieve gave this advice privately as attorney general last year when ministers agreed to adopt powers to remove passports from naturalised UK citizens who had the prospect of being a citizen of another state. He said the best course was to prosecute suspected terrorists in the UK courts.

Cameron said as many as 500 UK citizens were fighting in Syria or northern Iraq, representing the single greatest threat to the UK.

“It absolutely sticks in the craw that someone can go from this country to Syria, declare jihad, make all sorts of plans to start doing us damage and then contemplate returning to Britain having declared their allegiance to another state.”

With the US launching air attacks against Islamic State recently, Cameron refused to give any firm commitment on UK involvement, saying no such request had been received a formula being used by ministers until a clearer strategy emerges from Washington or is agreed at the Nato summit this week in Wales.

But Cameron signalled some flexibility when he said, in reference to air strikes: “We should ask ourselves how we best help those people on the ground who are doing vital work in countering [Isil]”.

He added that, as in Libya, “the British government must reserve the right to act immediately and inform the House of Commons afterwards”.