About 50 Labour MPs had been thought to be unhappy with the proposal
The parliamentary vote on extending terror detention to 42 days looks "very, very tight", Home Office minister Tony McNulty has said.
His comments to the BBC came after cabinet ministers were told there was "still more to do" to win on Wednesday.
Extending pre-charge detention from 28 days is opposed by the Conservatives, Lib Dems and 30 or so Labour MPs.
The result is expected to be so close that it might hinge on how the nine Democratic Unionist Party MPs vote.
To defeat the government, some 33 Labour MPs need to rebel, assuming all other MPs - including those of the DUP - also vote against it.
Liberal Democrat leader Nick Clegg urged the DUP to defy ministers.
He said: "The DUP has a proud record of defending civil liberties, most notably resisting internment, and I hope its MPs will remember that tradition when they vote on the government's proposals."
It seems that ministers or police have sexed up the dossiers for extension
Shami Chakrabarti Director
Police have claimed that, under the existing law allowing 28 days' detention before charge, they almost ran out of time to investigate two suspects over the failed plane plot at Heathrow in August 2006.
But on the eve of the vote, the pressure group Liberty told the BBC it had learned that the evidence used to charge the suspects had been obtained earlier.
Liberty's director, Shami Chakrabarti, said: "It seems that ministers or police have sexed up the dossiers for extension.
"I have learned that in two of the cases where people were charge late the evidence eventually used was available at four days and 12 days respectively."
Neither of the suspects mentioned by the group has yet gone to court.
Labour MP Keith Vaz, chairman of the Commons home affairs select committee, has written to his parliamentary colleagues, urging them to support the government.
Student Rizwaan Sabir says he 'felt terrorised' when held for six days
His letter said: "As a nation we need to stand together united and strong, giving no quarter to those who wish to destroy our way of life.
"We believe that these proposals strike the right balance between protecting lives and preserving liberties."
Mr McNulty told BBC Radio 4's World at One: "I think we are very hopeful but people shouldn't be under any illusion. It will be a very, very tight vote."
The opposition parties argue the proposed pre-charge detention limit would infringe civil liberties, but ministers say it is necessary to deal with increasingly complex terror plots.
In a sign of the sensitivities ahead of the vote, the head of the security service MI5 felt obliged to clarify its position on the proposal.
Jonathan Evans said the organisation was not best placed to advise because it was not responsible for prosecutions or criminal investigations.
Human rights are essentially just that - you can't just get rid of them if you think they are inconvenient
Trevor Phillips Equality and Human Rights Commission
He said the security services had not sought to comment on the measures publicly or privately, except to say they recognised the challenge posed to police by the increasingly complex and international character of terrorist cases.
Meanwhile, Scotland's Lord Advocate Elish Angiolini, who was appointed by the previous Labour Holyrood administration, told The Herald newspaper she was "not aware of any case [since 2000] where an extension of the period beyond 28 days would have been required".
And Trevor Phillips, head of the Equality and Human Rights Commission, said his organisation may seek a judicial review if the 42-days limit become law.
"Human rights are essentially just that - you can't just get rid of them if you think they are inconvenient," he said.
Amnesty International's UK director, Kate Allen, has written to all backbench MPs saying the vote represents a watershed for human rights.
Prime Minister Gordon Brown, who has a working majority of 65 in the Commons, has made it clear that he does not regard the vote as a matter of confidence in his premiership.
But a defeat would add to the pressure on him as he seeks to regain momentum after recent difficulties, poor polls and elections results.
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