Ulster Unionist MPs are split on the issue, but they could help the Government survive the vote by abstaining in protest at the Home Office going too far, as they see it, with the ban on handguns.
Willie Ross, one of the party's MPs, said: "This legislation is worse than the poll tax and the Dangerous Dogs Act. There has been no thought given to it whatsoever."
Without their nine votes, the Opposition parties fear they will be unable to force the Government to accept a total ban on handguns when the Firearms Bill is voted on in the Commons.
The Home Secretary is facing a growing backlash from Tory MPs who share the Unionists' view that the Government has gone too far. A group of Tory MPs who support shooting will seek an urgent meeting next week with Michael Howard, to seek exemptions in the proposed law for starting pistols for athletics, and the humane killers used by veterinary surgeons.
The Tory MPs opposing the Government's plans to ban handguns include Jerry Wiggin, Henry Bellingham and Michael Colvin. The Tory MP John Carlisle, who has also made it clear he opposes the legislation, was unavable for comment yesterday. "He has gone shooting," said his office.
Nicholas Budgen, the Tory MP for Wolverhampton SW, raised protests in the Commons warning that the arguments of the pro-gun lobby might be drowned out in the rush to pass a ban. Tony Newton, Leader of the Commons, told Mr Budgen that the general feeling in the House was that it was best to make "early progress" on the issue. But Mr Budgen added: "Speedy and particularly speedy and agreed legislation often leads to ill-considered and bad legislation, as certainly occurred with the Dangerous Dogs Act."
Labour will allow a free vote on its side, along with the Liberal Democrats. A few Labour MPs are expected to oppose a total ban and abstain or vote with the Government, reducing Labour's chances of victory.
If the Government wins the vote, it is expected that Labour will not oppose the third reading of the Firearms Bill.
Labour sources said last night that they expected to lose the key vote on the bill, which is likely to be held next month. "There is a majority in the Commons for a total ban but we can't win with the whip on the Tories," a senior Labour source said.
Labour, the Liberal Democrats, the SNP and Plaid Cymru are lining up to vote in favour of a total ban. They are expected to be joined by three Tory MPs - Hugh Dykes, Robert Hughes and the former Home Office minister David Mellor.
If the 17 Ulster MPs voted with the opposition, the three Tory rebels would be enough to defeat the Government by four votes. The four SDLP MPs are expected to vote with Labour but the Government is counting on some of the nine official Unionists led by David Trimble to rescue it from defeat.
Three Democratic Unionist Party MPs led by Ian Paisley may abstain but the independent Unionist, Robert McCartney, is expected to support the Government.