Ministers are facing a possible Labour rebellion over plans to renew the UK's nuclear weapons system.
The government could be forced to rely on Conservative votes
MPs will vote on Wednesday on the government's £20bn plans to replace the Clyde-based Trident system.
A BBC survey of Labour backbenchers found 64 out of 101 who responded opposed renewal. One ministerial aide has said he will resign over the issue.
But Defence Secretary Des Browne said he was "confident we will persuade people to come to our side".
Lack of support
The survey for the BBC's The World This Weekend found that out of the 101 Labour MPs who responded, 22 said they supported the renewal of Trident.
A total of 64 said they opposed it, and a further 15 were still undecided.
This could mean the government will have to rely on the votes of the Conservatives to carry Wednesday's motion to determine whether a new generation of nuclear submarines is acquired and the Trident D5 missiles updated.
Mr Browne told BBC One's The Politics Show the UK had an "obligation" to retain a deterrent as part of its membership of Nato.
He said: "It's not nearly as straightforward as people suggest. They sleep soundly in their beds at night because we have nuclear weapons."
Mr Browne added: "Some people feel they are prisoners of the position that the party had before it changed in the 1980s [which was one of unilateral nuclear disarmament]."
He went on: "I'm still confident we will persuade people to come to our side of the argument."
Lord Chancellor Lord Falconer told BBC One's Sunday AM show: "There will be people who take a different view, but I hope we demonstrate that we are broadly unified on the way to go forward."
He added: "I'm sure we will get very substantial support on the Labour benches."
Shadow defence secretary Liam Fox said the Conservatives were supporting a replacement for Trident to ensure the UK was "prepared for all eventualities".
He added: "We think the government's decision on Trident is the right one.
"We think we need to make an early decision for a number of reasons.
"And when the government is doing the right thing in terms of national security we think they should get the support of the opposition."
Liberal Democrat defence spokesman Nick Harvey said: "The Tories are making it too easy for Blair and Brown to put this issue to bed before the handover.
"I understand that the Tories support Trident's replacement, but surely even they can see that the final decision will not be made until 2012 -14. That's when Parliament should make its decision."
Livingston Labour MP Jim Devine, who is parliamentary private secretary to health minister Rosie Winterton, has indicated he will resign before the Commons vote.
Meanwhile, the Scottish National Party is urging ministers not to base the replacement in Scotland.
Leader Alex Salmond said Mr Devine's resignation would signal a Labour split.
"The government's Trident replacement policy will loom large over a divided Labour party in Scotland in May," he said.
Mr Salmond cited a government White Paper which states £1bn a year over a period of 15 years will have to be raised from the Treasury budget or from new taxes.
Labour leadership contender Michael Meacher said he would order a new vote on Trident after a "full and proper" public consultation if he won his bid to succeed Tony Blair as prime minister.
The former environment minister said the government was trying to "bounce" MPs into accepting a replacement.