Emily Thornberry risked reopening Labour divisions over the nuclear deterrent by indicating the party’s support for Trident could not be guaranteed.
Labour’s leadership has said Trident will be covered in a defence review if Jeremy Corbyn – a long-standing campaigner for nuclear disarmament – was in Number 10.
Labour’s policy, set by the party conference, supports Trident but shadow foreign secretary Ms Thornberry said she could not guarantee what the outcome of the review would be.
Ms Thornberry, who said she was “sceptical” about Trident, was asked in a radio interview to confirm that the missile system would remain as Labour policy after the review.
She replied: “Well no, of course not, if you are going to have a review, you have to have a review.”
On LBC Radio she said: “Overwhelmingly we need to make sure that our policy is up to date and meets 21st century threats and no one can disagree with that, surely?
“You know there was a time when we gave up on sabres or horses, you need to keep updating your defence policy and meeting the most pressing and most obvious needs.
“We need to make sure any war that may be fought, will be fought on 21st century terms.”
Asked if she was pushing for a change in the policy of supporting Trident, she said: “I think that what we should do is we should have a modern policy on defence and we should make sure first and foremost that Britain is safe.
“That is the first and overwhelming obligation of any government, that is where we start.”
Party sources had previously insisted said that although the deterrent would be covered by the review, the renewal programme to replace the submarines carrying the Trident nuclear missiles was not in question.
Labour’s manifesto published on Tuesday included support for the nuclear deterrent.
“Labour supports the renewal of the Trident nuclear deterrent,” the document said. “As a nuclear-armed power, our country has a responsibility to fulfil our obligations under the Nuclear Non-Proliferation Treaty. Labour will lead multilateral efforts with international partners and the UN to create a nuclear-free world.”
Ms Thornberry said: “How much does the public feel that their safety and security is dependent on us having a nuclear deterrent? And how much are we a safe world by having so many countries with nuclear deterrents?
“Because in the end, if countries continue to have nuclear deterrents then we are actually making our world less safe, we are making it more likely that there will be a nuclear attack.”
Senior Tory Bob Neill said: “Jeremy Corbyn and Emily Thornberry want to disarm Britain and put our security at risk. Corbyn’s coalition of chaos would scrap Trident, abandon our allies and would rather talk to Daesh than strike its barbaric leader.
“Only Theresa May and the Conservatives will provide the strong and stable leadership needed to defend our values against those who seek to destroy our way of life.”