An influential group of MPs and peers has rejected the government's assurances about its amendments to the 42-day pre-charge detention proposals.
The joint committee on human rights said the amendments were "inadequate" to meet their concerns.
In a seperate development the Church of England has confirmed it will lead opposition to the proposals in the House of Lords.
In a letter to MPs, the church said: "We believe any extention beyond 28 dyas will unacceptably disturb the balance between the liberty of the individual and the needs of national security."
The two rebuttals will disturb Jacqui Smith, the home secretary, who had good reason to think her talk to Labour MPs on Monday had secured a victory on next week's vote.
As things stand, the vote hangs on a knife edge, with the main variable being whether rebel Labour MPs will put their principles above further weakening the government.
The committee decided the trigger for 42-day detention - a "grave exceptional threat" which could occur domestically or from overseas - fell short of their preferred terminology: "A public emergency threatening the life of the nation".
It cast doubt on the effectiveness of allowing parliamentary approval of the home secretary's exercise of the power, because any debate would be severely limited by the risk of prejudicing a future trial.
Government claims of significantly strengthening judicial oversight were treated with contempt by the committee, who said simply: "The government has not proposed any additional judicial safeguards for individuals against arbitrary detention, despite the committee's repeated warning that the lack of a fully judicial hearing amounts to a breach of the suspect's rights."
Finally, the MPs were unimpressed by the reduction of the power's implementation from 60 to 30 days, saying such a change meant nothing because the reserve power can be indefinitely renewed.
Andrew Dismore, chair of the committee, said any extension to pre-charge detention would be illegal.
"The proposed extension to 42-days would almost certainly not be lawful," he said.
"The government has talked of a major emergency, the 'nightmare scenario' of simultaneous plots across Britain or two 9/11s at once; yet the amendments tabled by the government provide for possible events falling well short of that.
"We do not underestimate the seriousness of the threat this country faces from terrorism. However, in the case of a genuine 'emergency threatening the life of the nation', the law already provides the right to derogate from the right to liberty," he concluded.
Liberal Democrat Dr Evan Harris, who is also on the committee, said: "This report demonstrates why no-one opposed to 42 pre-charge detention can be in any way reassured or persuaded by the government's proposed amendments; laying bear as it does continuing lack of the basic safeguards required.
"As such it is required reading for Labour backbench potential rebels."
Commenting on claims the new power would illegal, a home office spokesperson told politics.co.uk: "We're satisfied the proposals fully comply with article five of the European Convention of Human Rights. There has been no case where our detention of a terrorist suspect has been found incompatible or unlawful with that article."
She continued: "We're still considering the advice given by the committee, however our view is that reasonable suspicion is indeed required for detention, and the courts will have to be satisfied of this.
"We also remain of the view the increased length of detention would be proportionate given the various statutory tests used."