Some of the countries most influential peers have launched a devastating attack on the government plans for 42-day detention, saying they could compromise the independence of the judiciary and place undue influence on parliament.
The House of Lords constitution committee, which examines the constitutional implications of government bills said allowing MPs to vote on an individual's incarceration forced parliament to make decision it is "institutionally ill-equipped to determine".
"Considering that any debate will be highly political in nature and any vote may well be whipped by the political parties, we are deeply concerned that the independence of the judiciary may appear to be undermined and that trials may be prejudiced," said Lord Goodlad, committee chairman.
The ability of MPs to vote on each individual case of extended detention was originally intended as a concession to potential rebels. Home secretary Jacqui Smith said allowing MPs a vote ensured the powers were subject to parliamentary scrutiny.
But the lords committee said it "risks conflating the roles of parliament and the judiciary, which would be quite inappropriate".
There are also concerns the bill could threaten the right to a fair trail, with peers concerned judges would have to determine applications just hours after a highly politicised debate in parliament.
"This is a recipe for confusion that arguably risks undermining the rights of fair trial for the individuals concerned," the committee said.
Liberal Democrat home affairs spokesperson David Howarth said: "The 42-day proposal has everything to do with politics and nothing at all to do with the struggle against terrorism.
"I'm glad this committee has helped to expose ministers' misleading claims."
Dominic Grieve, shadow home secretary, said: "The committee joins a long list of experts who have condemned the government's ill-thought through proposals for 42-days as both unfair and unworkable."