Yesterday, the Labour leader suggested Trident renewal might not be in Labour’s election manifesto – only to be corrected within hours by party colleagues.
Speaking to BBC Radio Four’s Today programme, Mr Fallon said voters tempted by Labour had been left “completely unsure as to what would actually happen to our nuclear deterrent”.
But he went further, marking out a clear divide between the parties when asked if Ms May was ready to use Trident as a “pre-emptive initial strike”.
“In the most extreme circumstances, we have made it very clear that you can’t rule out the use of nuclear weapons as a first strike,” Mr Fallon said.
Asked in what circumstances, he replied: “They are better not specified or described, which would only give comfort to our enemies and make the deterrent less credible.
“The whole point about the deterrent is that you have got to leave uncertainty in the mind of anyone who might be thinking of using weapons against this country.”
Mr Fallon also insisted that critics of Trident – including senior military figures who have ridiculed the idea that it is an effective deterrent – were “absolutely wrong”.
“It deters day and night every single day of every single year,” the Defence Secretary insisted.
Mr Corbyn sparked fresh Labour despair when, asked if Trident renewal would be in Labour’s manifesto, he replied: “We haven’t completed work on the manifesto yet.”
Last year, the Labour leader had appeared to abandon attempts to persuade his party to back unilateral disarmament, after a conference vote in favour of Trident.
Mr Corbyn also refused to say whether he would order the captains of the UK’s nuclear submarines to launch their missiles if the Government had been wiped out by a nuclear strike.
Today, Labour’s general election chief, Andrew Gwynne, insisted renewing Trident will not be part of Labour’s defence review if it wins the general election.
“We are committed to renewing the Trident system,” said Mr Gwynne – rejecting Mr Corbyn’s statement that “all aspects” of defence policy were up for grabs.
“The Labour party is very clear we are committed to a credible nuclear credibility at the minimum end of the scale. That is Labour party policy and it will be in the manifesto,” Mr Gwynne said.
But he appeared to rule out a first strike, adding: “We would not be in a position where the first choice would be to press that red button. It is a deterrent because we have them.
“We believe in multilateralism, we believe in negotiating away our nuclear weapons system to create a nuclear weapon free world.”
But Mr Fallon said, of Mr Corbyn: “He’s against the nuclear deterrent, would stop building the submarines which we have already started building, he wouldn’t control our borders – and, earlier, he has even questioned our Nato deployment.”