Renewing the UK's nuclear deterrent is not the right priority for the Ministry of Defence, the House of Commons has been told.
SNP Westminster leader Angus Robertson, leading an opposition day motion on the future of Trident, said investing billions on it was not justified.
A call to scrap the proposed replacement for the Trident nuclear weapons system was rejected by MPs.
The Commons voted against it by a margin of 364 to 35.
Defence Secretary Michael Fallon said for security reasons it was right that the UK had a nuclear deterrent.
Next year MPs will have to decide whether to authorise construction of new submarines to carry Trident.
The current generation of four subs would begin to end their working lives some time in the late 2020s.
The SNP, Plaid Cymru and the Greens want to see Trident scrapped. The Conservative Party is committed to a like-for-like replacement; Labour says it supports a continuous at-sea deterrent and the Liberal Democrats are keen to look at alternatives.
Mr Robertson told the Commons: "The time has come to put down a marker about scrapping Trident and not replacing these weapons of mass destruction.
"I've yet to hear a supporter of Trident convincingly explain in what circumstances they are prepared to justify the killing of hundreds of thousands of innocent men, women and children and cause massive environmental damage to the world for generations to come.
"That is a consequence of the use of nuclear weapons and surely if one has them, one has to be prepared to use them. I've yet to hear anybody give an example of circumstances where they are prepared to kill millions and millions of people."
He added: "The point is that the MoD has the wrong priorities, investing billions on nuclear weapons which it can never use, but not properly managing the conventional armed forces which are so necessary."
However, Tory coalition minister Mr Fallon insisted that the government "could not gamble with our country's national security".
He said: "We have to plan for a major direct nuclear threat to this country or to our Nato allies that might emerge over the 50 years that the next generation of our submarines will be in service.
"In a world that is getting more dangerous there are no alternatives that offer the level of protection and security that this country needs."
Mr Fallon raised the examples of Russia, North Korea and Iran and stressed the world could not simply "dis-invent" the some 17,000 nuclear weapons in circulation globally.
During the debate Labour's Vernon Coaker told MPs that is was "right for the UK to maintain a credible, minimum independent nuclear deterrent based on a continuous at-sea posture".
He added: "But what we have said of course as part of the Strategic Defence and Security Review that we will look to see whether a continuous at-sea deterrent can be delivered in a more cost effective way."
Schools and hospitals
Plaid Cymru MP Jonathan Edwards - whose party co-signed the SNP motion against Trident renewal - said renewal would cost an estimated £100bn over its lifetime.
He went on to question how this could be justifiable when schools and hospitals were "crying out for investment".
Liberal Democrat MP, Sir Nick Harvey, suggested the way forward was for the component parts of a nuclear weapons system to be kept and assembled if needed.
Mr Fallon rejected that approach saying a part-time deterrent would not work.