Britain's House of Lords rejected key parts of Prime Minister Tony Blair's terrorism bill today.
Current powers to keep foreign terrorism suspects in jail without trial expire on Monday, after which 11 men, mostly in London's top-security Belmarsh prison, could walk free.
But the battle to replace the old laws with controversial new powers - including house arrest - has raised political controversy over rights and freedom and has fast become a big issue in the run-up to Britain's expected May election.
Mr Blair has won over House of Commons rebels in the Labour party ranks with major concessions. But the Lords, which has a Conservative majority, rejected key parts of the bill again today, raising the prospect of a series of "ping-pong" votes between the two chambers.
Mr Blair has conceded that judges rather than politicians will have the final say on all forms of "control orders" - up to and including house arrest - on terrorism suspects, and that parliament will review the law annually.
But he has refused opponents' demands for a "sunset clause" in the bill whereby it would lapse entirely in November, allowing legislators to start from scratch.
The Lords voted 250 to 100 to reinstate the sunset clause, albeit at the later date of March next year, defying the convention that they do not repeatedly flout the view of the lower, more powerful chamber.
At the heart of the controversy are the Belmarsh prisoners. "On Monday, if this (bill) is not passed, these men will be released. I think that would be a grave threat to national security," Sir Ian Blair, head of London's Metropolitan Police, told reporters.
Britain's highest court ruled late last year against the existing powers, which allow for the indefinite detention of foreign terror suspects. Those powers lapse on Monday.