Labour will not support Government moves to increase the number of nuclear warheads in its stockpile, according to Lisa Nandy.
UK ministers previously committed to reducing the level to a maximum of 180 by the middle of the decade, but now the stockpile could be up to 260.
Shadow foreign secretary Ms Nandy said ministers have yet to justify a decision which has “baffled” the Opposition and others.
She added Labour “won’t support” the decision until it can be explained by the Government, suggesting it will either abstain or vote against the measure.
Asked if Labour would support another 80 nuclear weapons, Ms Nandy told BBC One’s Andrew Marr Show: “We are absolutely baffled, as many Tory MPs are as well, about why the Government has chosen this moment – at the point at which the United States has stepped forward to try to deal with nuclear proliferation, signing a new treaty with Russia, at the point we face a growing threat from Iran from nuclear weapons – to do it.
“There may be a reason why they’ve done this.
“One of the examples mooted has been perhaps they need to have two nuclear deterrents concurrently.”
Pressed to answer the question, Ms Nandy replied: “This is a really serious step that potentially threatens the security of our country.
“So far the Government hasn’t given any reason why it’s broken with 50 years of convention and gone against the direction of travel that reigning in nuclear weapons is an important part of our safety and security.
“Until they can give an explanation to the House of Commons, we won’t support them.”
The Government last week published details of its major review of foreign and defence policy, known as the Integrated Review.
It stated the UK could consider deploying its nuclear arsenal against non-nuclear countries if they possess equivalent weapons of mass destruction – including new “emerging technologies”.
While the document stated the UK will not fire, or threaten to use, its missiles at a non-nuclear state, it also says assurance could be reviewed in future in order to maintain the deterrent effect against potential adversaries.
The UK’s policy is to only consider using its nuclear weapons “in extreme circumstances of self-defence, including the defence of our Nato allies”, according to the Integrated Review of Security, Defence, Development and Foreign Policy.
Defence Secretary Ben Wallace said the Government is lifting the cap to ensure the UK has a “credible” nuclear deterrent in response to the actions of Russia and others.
He told the same programme: “We have seen Russia invest strongly in ballistic missile defence. They have planned and deployed new capabilities, and that means if it’s going to remain credible it has to do the job.
“We’ll still be the lowest, by the way, by a long short. They have a stockpile of 300.
“Quite a clear study of effectively how warheads work and how they re-enter the atmosphere means that you have to make sure they’re not vulnerable to ballistic missile offence, otherwise they’re no longer become credible.”