The government says more time is needed to foil complex plots
Ministers have been urged to drop proposals to extend the limit on holding terror suspects without charge, by a second Parliamentary committee.
A day after an MPs' committee ruled there was no evidence to support an extension from 28 to 42 days, a joint committee of MPs and peers has agreed.
The joint committee on human rights (JCHR) also said proposed safeguards in the plan would be "virtually useless".
Home Secretary Jacqui Smith said she would continue to seek a consensus.
The JCHR report comes a day after the home affairs committee ruled that ministers had not made a convincing case for extending the limit.
'Murder and mayhem'
Ms Smith says, owing to the increasing complexity of terrorism cases, police may need to hold someone longer than 28 days in order to "prosecute people who want to cause murder and mayhem on our streets".
But plans to extend the limit before have proved controversial and moves to extend it to 90 days in 2005 were defeated by a combination of Tory, Lib Dem and some Labour MPs.
The home secretary has been praised for her approach in trying to seek a consensus on extending the limit.
But JCHR chairman Andrew Dismore, a Labour MP, told the BBC: "There is no consensus that there should be any extension beyond 28 days - there's no evidence to support this and 28 days has proved to be entirely sufficient so far.
"There is a consensus about alternatives, such as post-charge questioning, the use of intercept evidence, charging people on a much lower threshold of reasonable suspicion and the use of new offences which are much easier to show - such as acts preparatory to terrorism."
His committee's report said the government had "failed to consider" how to use the alternatives to avoid the risk of investigation teams running out of time.
The home affairs committee report also favoured other legal changes - such as allowing "intercept" evidence like phone taps in court.
Ms Smith's latest proposals would involve the House of Commons and the House of Lords approving an extension to 42 days in exceptional cases, within 30 days.
But the JCHR said that would be "virtually useless as a safeguard" as the delay would mean a suspect could end up being held for 42 days anyway, even if Parliament eventually voted against it.
It might also jeopardise any future trial, the committee said.
And the fact that the Crown Prosecution Service was not calling for more time, was "devastating" to Ms Smith's argument, it found.
Mr Dismore urged the government to "drop this ill-conceived proposal".
In response, Ms Smith said she would "look carefully" at the committee's report and continue to seek a Parliamentary consensus on the proposals for "a temporary extension in exceptional circumstances" of the 28-day limit.
But she said: "I have not been convinced that measures on post-charge questioning and the use of intercept as evidence would in themselves be sufficient."
She said the home affairs committee had noted there was a "real and acute" threat facing the UK.
"As home secretary I am not willing to leave this potential risk to the security of the British people unaddressed," she said.
Eric Metcalfe, of the human rights group Justice, said the report suggested the government was heading for another Commons defeat on its plans.
"The committee's report is another nail in a coffin that the government should never have exhumed in the first place," he said.