Labour will not support Government moves to increase the number of nuclear warheads in its stockpile, according to Lisa Nandy.
Despite a push in recent years to reduce the nuclear stockpile to a maximum of 180, recent policy changes now mean it could be up to 260.
Shadow foreign secretary Ms Nandy had said Government decision-making on the issue had left the Opposition and others baffled.
She added that Labour "won't support" any decisions, by abstaining or voting against any measures until an explanation is given.
Ms Nandy was asked on BBC One's Andrew Marr Show whether Labour would support the addition of 80 nuclear weapons.
She said: "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."
When pushed 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 long-awaited Integrated Review, was published on Tuesday, and has been billed as the most radical reassessment of Britain's place in the world since the end of the Cold War.
It stated the UK could consider deploying its nuclear arsenal against non-nuclear countries if they possess equivalent weapons of mass destruction – a list that included "emerging technologies".
The UK's policy is to only consider using them "in extreme circumstances of self-defence, including the defence of our NATO allies".
Defence Secretary Ben Wallace spoke on the same programme about the decision, arguing that the growth in number was needed to keep a "credible" nuclear deterrent.
"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," he said.
"We'll still be the lowest, by the way, by a long shot. 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 no longer become credible."
Cover Image: MOD