To defeat the government, some 33 Labour MPs need to rebel, assuming all other MPs - including those of the DUP - also vote against it.
In the Commons debate on the Counter-Terrorism Bill, Home Secretary Jacqui Smith said the "reserve power" for 42 days was necessary to counter a threat that was "more ruthless than we have ever faced before".
The government was "not proposing an automatic or immediate extension to pre-charge detention beyond 28 days", she said.
It would only be used with the support of the director of public prosecutions, the backing of Parliament in a vote, with judicial safeguards and only for a temporary period.
She confirmed that suspects held beyond 28 days but then released without charge would be eligible for an "ex gratia" compensation payment.
Liberal Democrat leader Nick Clegg urged the DUP to oppose the measure. "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," he said.
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.
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.
Mr 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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