The SNP has pressed its call for Trident nuclear weapons to be scrapped as Labour MPs prepared to defy Labour leader Jeremy Corbyn over the deterrent.
Defence spokesman Brendan O'Hara moved the SNP opposition day motion surrounded by scores of party colleagues but facing few Labour and Tory MPs around the Commons.
Most of those Labour MPs present - in Parliament despite orders from their leadership to stay away amid claims the debate was a "stunt" - were prominent supporters of Trident.
Mr O'Hara said he was disappointed Mr Corbyn's MPs had not fallen in line with the Labour leader's firm opposition to Trident.
Official Labour policy backs Trident and the planned replacement of the Vanguard-class submarines carrying the missiles. Scottish Labour voted at its recent conference to abandon the weapons.
Mr O'Hara told MPs: "I have always argued there is no moral, economic or military case for Trident. Let's be absolutely clear there is absolutely no moral case for any state possessing weapons of mass destruction - possessing the wherewithal to destroy the world several times over and everything in it is not something to be proud of, indeed, it is something I believe to be deeply ashamed of.
"Not only is Trident morally questionable, I think it is economic madness. Back in 2006 when the successor programme was first discussed, the likely cost of building new subs was put at between £15 and £20 billion. Yesterday's SDSR put it at £31 billion, with on top of that a £10 billion contingency."
He added: "Trident is not a military weapon. Trident is a political weapon - and it is a political weapon which can never and will never be used except to consume anything between 30% and 50% of the UK defence procurement budget."
Mr O'Hara said it was a "military and political ego trip" adding the money could be better spent peacekeeping, reacting to emergencies such as the Ebola crisis, or helping refugees in the Middle East.
He continued: "In many ways, to put it in a more colloquial way, we are acting as having a fur coat and nae knickers... it is being paid for on the backs of the poor."
Mr O'Hara accused Labour of hiding behind "the fig leaf of multilateralism" and said he had no doubt that Mr Corbyn would join the SNP in voting against Trident renewal next year.
He said: "A forlorn hope indeed, but there was a genuine hope, that perhaps with the election of Mr Corbyn that there would be at least a debate on Trident in this place.
"I fear that the leader has not managed to take his party with him and the paltry attendance today from the Labour Party would suggest exactly that."
Internal Labour tensions over the issue exploded into public view last week when it was announced Mr Corbyn had appointed former London mayor Ken Livingstone as a "co-convener" of a party defence review alongside shadow defence secretary Maria Eagle, who supports Trident.
Shadow chancellor John McDonnell blasted the Commons debate as an "SNP stunt" following a meeting of Labour MPs in Parliament last night
Former cabinet minister Ben Bradshaw (Exeter) said on Twitter he would defy the order to abstain, while other MPs, including John Woodcock (Barrow and Furness) and Jamie Reed (Copeland) were in the Commons and expected to vote against the SNP motion in defiance of Mr Corbyn.
Prime Minister David Cameron announced yesterday the cost of replacing the ageing Vanguard-class subs could rise to £41 billion.