Prime Minister David Cameron was to outline tougher measures against jihadist suspects 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 measures could include making it easier to strip suspected would-be jihadists of their passports in Britain and giving more data on airline passengers to the intelligence services.
Britain raised it terror threat risk level to "severe" on Friday due to fears over the situation in Iraq and Syria.
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 jihadist group Islamic State (IS).
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.
"At the very least it's the kind of question which will be tested here in our own courts and perhaps also in the European Court of Human Rights."