Prime Minister Tony Blair could face one of the biggest rebellions by his Labour Party since the start of the Iraq war, as MPs will vote on the £20bn plan to renew the UK's nuclear weapons system.
The Commons debate on replacing the submarines which carry the Trident missiles is due to start at about 1240 GMT, with a vote due near to 1900 GMT, according to BBC.
About 80 Labour MPs have expressed unease, but defeat is unlikely because the Conservatives back the plans.
Mr Blair claims the UK needs nuclear weapons in 'an uncertain world'.
Meanwhile, church leaders from across the denominations have reaffirmed their united opposition to Trident ahead of the debate.
The Bishop of Southwark, the Rt Rev Tom Butler, called on MPs to take into consideration the concerns of the Church of England that upgrading Trident would conflict with the "spirit of the UK's obligations in international law and the ethical principles underpinning them".
The Moderator of the General Assembly of the Church of Scotland, the Rt Rev Alan McDonald, co-signed a letter encouraging all MPs to reject the renewal of Trident along with Cardinal Keith Patrick O'Brien, chairman of the Catholic Bishops Conference of Scotland, and Dr Idris Jones, the Primus of the Scottish Episcopal Church.
Rev McDonald said: "£20 billion is an obscene amount to spend on nuclear weapons, but even if a Trident replacement cost nothing, we should still have nothing to do with it. Because it's wrong, morally, and theologically wrong."
"As Disciples of Christ, our calling is to be peacemakers in the world. There can be no place for weapons of mass destruction in the world that God loves so much."
