Jo Swinson has been criticised for trying to pass a “virility test” after saying she would be willing to press the nuclear button if she becomes prime minister.

The Liberal Democrat leader was asked if she would “ever be prepared to use a nuclear weapon”, answering with a single word: “Yes.”

The Campaign for Nuclear Disarmament attacked the “disgraceful response”, saying: “Not even a moment’s hesitation about the prospect of killing millions of people. We need better than this.”

And Nicola Sturgeon, the Scottish National Party leader, said: “It’s sickening to hear this question asked and answered as if it’s some kind of virility test and without any context.

“Using nuclear weapons would mean killing millions of people. Those consequences should be made clear.”

But a Lib Dem spokesman defended Ms Swinson’s answer, saying: “We support multilateral disarmament, but if you have a nuclear deterrent and tell everyone you won’t use it, it ceases to be a deterrent.”

The controversy comes after Labour got into difficulty over its stance on the UK’s Trident nuclear submarines. It is party policy to retain them – but Jeremy Corbyn has previously said he would not use them.

Emily Thornberry, the shadow foreign secretary, said Labour would “work collectively” on how to respond to a nuclear threat.

The Lib Dem general election manifesto is expected to back maintaining “a minimum nuclear deterrent”, while pursuing international talks to achieve multilateral disarmament.

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When in office, in the Cameron-Clegg coalition, the party pledged to put forward alternatives to scale down the Trident fleet, but the idea came to nothing.

Defence experts have suggested that the number of Vanguard submarines could be reduced, ending so-called ‘continuous at sea deterrence’, without putting the UK at risk.

However, little has been heard about the idea from the Lib Dems since Nick Harvey was sacked from his defence post in the coalition in 2012.

On the campaign trail, Mr Corbyn has also declined to rule out scrapping Trident as part of any post-election arrangement with the SNP.

The Labour leader said that, as prime minister, he would seek to revive non-proliferation talks with other nations, with the UK’s nuclear weapons on the table.

Asked whether he would agree to scrap Trident if the SNP insisted on that as the price of backing a Labour government in a hung parliament, Mr Corbyn said: “I think the SNP would actually agree with me, and indeed in the past they certainly have, that the priority has to be giving realism to the nuclear non-proliferation treaty.”