The defence secretary, Michael Fallon, has portrayed Jeremy Corbyn as a security risk as Labour’s divisions on nuclear weapons continue to be exposed.

Over the weekend the Labour leader defied his own party’s policy on Trident, forcing senior members of the shadow cabinet on Monday to insist that a Labour government would fire nuclear weapons in some circumstances.

Speaking on the BBC2’s Daily Politics, the shadow defence secretary, Nia Griffith, contradicted her leader by saying: “We are prepared to use it, and I’m certainly prepared to use it.”

She added: “Everybody wants to negotiate first … But if you have the option of force – conventional forces initially, but you have the ultimate deterrent as well – then you are far more likely to sustain peace and security at the end of the day.”

Earlier Fallon said a Conservative government would be prepared to use nuclear weapons. “In the most extreme circumstances you can’t rule out the use of nuclear weapons as a first strike,” he told BBC Radio 4’s Today programme.

But Fallon refused to specify those circumstances, adding: “The whole point about the deterrent is that you have got to leave uncertainty in the mind of anybody who might be thinking of using weapons against this country.”

Asked about a string of senior military figures who regard Trident useless as a deterrent, he said: “You will often find some military figures who will prefer to spend more money on conventional weapons than on nuclear … but it is better to have the deterrent because you cannot be sure that … nobody might use a weapon against us.”

Speaking on BBC1’s the Andrew Marr Show on Sunday, Corbyn had incorrectly suggested renewing Trident would be part of Labour’s defence review. The party’s general election chief, Andrew Gwynne, confirmed this was not Labour policy. “We are committed to renewing the Trident system,” said Gwynne, who is a shadow minister without portfolio.

“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,” he told BBC Radio 4’s Today programme on Monday,

Fallon also seized on Labour’s confusion and Corbyn’s equivocal answers on military decisions, including saying he would want to know what could “be achieved” by ordering a drone strike if the intelligence services located the whereabouts of the leader of Islamic State.

Fallon said: “Yesterday we had the staggering performance of somebody who wants to be prime minister saying he wouldn’t necessarily authorise strikes against terrorists. 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.”

Fallon praised “brave Labour MPs” who backed the renewal of Trident, adding: “Jeremy Corbyn made it absolutely clear that he is still against the deterrent, and he was then corrected by his own party. So we had chaos from Labour last night, which doubles the security risk to this country when you have somebody standing to be prime minister who his own party is then having to correct. And you are left completely unsure as to what would actually happen to our nuclear deterrent.

“This is somebody who would certainly put the security of our country at risk. And if you want stronger and stable leadership then it has to be Theresa May and the Conservatives.”