LONDON  - Prime Minister David Cameron was to outline tougher measures against militant suspects on Monday after Britain raised its security risk assessment to a level where an attack is thought “highly likely”.

Cameron was to give a statement to the House of Commons after 3:30 pm (1430 GMT) on fresh steps against suspects when there is insufficient evidence to charge them with a crime.

British media reported that the measures could include a “temporary bar” on Britons suspected of fighting in Syria and Iraq from returning home. Other steps could include making it easier to strip suspected would-be militants of their passports in Britain and giving more data on airline passengers to the intelligence services.

Britain raised its terror threat risk level to “severe” on Friday due to fears over the situation in Iraq and Syria. Some 500 British militants are estimated to be fighting there. The move, which means an attack is considered “highly likely”, came after the killing of US journalist James Foley, apparently by a man with an English accent who belonged to the militant group Islamic State (IS), formerly known as ISIL.

The threat level is now at the second highest out of five possible categories, its highest since July 2011.

Cameron has warned that the advance of IS raises the prospect of “a terrorist state on the shores of the Mediterranean.” “What we’re facing in Iraq now with ISIL is a greater and deeper threat to our security than we have known before,” he said at a Downing Street press conference Friday.

The centre-right Conservative prime minister was facing a struggle to persuade his coalition partners the Liberal Democrats to back his plans.

Negotiations were reportedly still going on Monday morning, just hours before Cameron was due to deliver his statement.

Civil liberties are a key part of the centre-left Liberal Democrats’ political philosophy and the party will be reluctant to back steps it sees as too draconian ahead of next year’s general election.

In an indication of the unease felt by some, former Liberal Democrat leader Menzies Campbell, a member of Parliament’s Intelligence and Security Committe, said it could be illegal to stop British citizens returning home.

“To render a citizen stateless is regarded as illegal in international law. To render them stateless temporarily, which seems to be the purpose of what’s been proposed, can also, I think, be described as illegal,” he told the BBC.

Another former Liberal Democrat leader, Paddy Ashdown, who was also international high representative for Bosnia and Herzegovina, warned against a “knee-jerk reaction.”

“It is always right for politicians who value liberty to resist attempts to increase arbitrary executive powers unless this is justified, not by magnifying fear, but by actual facts,” he wrote in Sunday’s Observer newspaper.

Britain has grappled for years with how to deal with people suspected of terrorism where there is not enough evidence to prosecute or deport them.

After the September 11, 2001 attacks in the US, Tony Blair’s government brought in a power permitting foreign terrorism suspects to be held in prison without charge or trial.

This was ruled unlawful in 2005, prompting ministers to introduce control orders, which allowed a wide range of movement restrictions on people suspected of involvement in terrorism.

Cameron’s government announced in 2011 that these would be scrapped and replaced with Terrorism Prevention and Investigation Measures (TPIMs), following criticism that control orders were too restrictive.

TPIMs allow suspects who have been assessed by intelligence agencies to be tagged electronically, banned from certain places and prevented from travelling overseas.

There are currently none in force, although two suspects subject to TPIMs, Ibrahim Magag and Mohammed Mohamed, absconded in 2012 and 2013 respectively.

Somali-born Mohamed, suspected of connections to Al-Qaeda linked Shebab, went missing after changing into a burqa at a mosque in west London and slipping away.