Opponents of the government plans to extend pre-charge detention to 42-days are vowing to take their struggle to the Lords, following last night's paper-thin victory for the government.
The second chamber is widely expected to reject the proposals and there are concerns in Labour ranks that more backbenchers could rebel against the party should the plans return to the Commons.
The government won last night on the basis of nine votes from DUP members, with 36 Labour MPs rebelling. That substantial number could mean they are joined by more doubters next time, in the assurance they will not be alone.
Kate Allen, director of Amnesty International UK, said: "Amnesty will now take the fight to the Lords.
"These are important human rights that are being taken away and we will do all we can to protect them. People have a right to be charged promptly or to be released: it's shocking that the law in the UK is moving further and further away from this basic principle."
Director of Liberty, Shami Chakrabarti, told politics.co.uk: "We fight on in the House of Lords.
"People have different views about the Lords but if it has one purpose it's as a backstop to protect our fundamental rights and freedoms when the hubbub of politics lets us down."
The result last night, carried by 315 to 306, resulted in almost unprecedented scenes in the chamber. Once MPs realised the result hung on DUP votes Lib Dem and Tory parliamentarians turned to the DUP benches to shout: "You've been bought."
Speaker Michael Martin desperately tried to bring the situation under control, censoring the chants with an angry exchange calling on no member to question someone's intention when voting.
Despite DUP claims to have voted on the basis of national security, numerous incentives are being gossiped about in the Westminster village, including extra financial support for Northern Ireland.
Apart from the DUP, almost every other party in the Commons voted against the government, including six SNP MPs and three Plaid Cymru MPs.