An influential parliamentary group claims there is not enough evidence to justify extending the time a terrorist suspect can be held from 14 to 28 days.
The Terrorism Bill returns to the House of Lords on Monday
Last month Tony Blair suffered his first ever defeat when MPs threw out plans to allow police to hold suspects for up to 90 days without charge.
Instead MPs voted to extend the detention time from 14 to 28 days.
But the Joint Committee on Human Rights says this could still pose a potential legal problem for the government.
The committee, which is made up of MPs and peers, says the plan could lead to the "inadmissibility at trial of statements obtained following lengthy pre-charge detention".
But the group does not rule out the possibility that such evidence might be produced in the future, subject to improvements in safeguards for the detainee being made.
The issue is likely to be scrutinised on Monday when the Terrorism Bill returns to the Lords for its line-by-line committee stage.
The Joint Committee on Human Rights says it is also concerned that the new offence of glorifying terrorism is "not sufficiently legally certain".
But Lords leader Baroness Amos says under the plans prosecutors will have to show there is a "deliberate intent" in order to bring about charges.
The committee says extending the grounds on which an organisation could be banned to include groups which "glorified" terrorism could conflict with the right of freedom of expression and freedom of association.
It suggests that there be a "reasonable excuse" or "public interest" defence to the proposed new offence of training for terrorism.
Lady Amos says deliberate "intention" to "glorify or incite terrorism" will be built into the bill.
"What we are seeking to do is to strengthen that part of the bill and make the tests stronger," she said.
Home Secretary Charles Clarke is also considering representations from organisations such as libraries and universities which are concerned that elements of the legislation could affect the right to free expression, she said.
Librarians are worried about lending material which might be construed as having details about terrorism.
There are also question marks over whether the academic study of the Middle East could be interpreted as the "encouragement" or "glorification" of terrorism.
"We may well come back with some amendments to deal with that point," said Lady Amos.
But she was not keen to comment on calls by the Association of Chief Police Officers (Acpo) to reconsider plans to ban extremist organisations.
The police have warned that the measure could drive such groups underground, making it more difficult to monitor them.
She said ministers had to strike a balance between police concerns and those of "others working in the field" who felt that the legislation should go further.