© David Moir / Reuters HMS Vengeance, a British Royal Navy Vanguard class Trident Ballistic Missile Submarine.
"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," Fallon told the BBC's Today program.
When 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."
The prime minister's official spokesperson later added there was "no reason to disagree with what the defense secretary said."
Trident's four submarines operate a continuous at-sea nuclear deterrent. Last year, a vote in the House of Commons saw MPs vote for Trident's renewal, which is expected to cost up to £225 billion (about US$285 billion) over its service lifetime.
Fallon's comments come as the Tories continued to exploit Labour divisions on the retention of the Trident deterrent, to warn of the "very dangerous chaos" if Jeremy Corbyn becomes prime minister.
On Sunday, the Labour leader said he would never authorize the use of nuclear weapons and suggested Trident renewal might not be in Labour's election manifesto - only to be corrected within hours by party colleagues.
Speaking to the BBC, Fallon said voters tempted by Labour had been left "completely unsure as to what would actually happen to our nuclear deterrent."
"I think you saw Jeremy Corbyn yesterday questioning strikes against terrorists, refusing to back the nuclear deterrent, he's been querying our NATO deployment and he seems to have fallen out with his own party over nuclear deterrent.
"That's chaos, but it's very dangerous chaos that would put the security of our country at risk."
Corbyn, a long-standing proponent of total nuclear disarmament, believes a Trident renewal is expensive, unsafe, ill-suited for contemporary warfare and in violation of international commitments.
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," he said.
Reader Comments
So a nuclear strike, destroying untold people, untold infrastructure that has taken decades if not centuries to create through blood sweat and toil.
And it's all destroyed by the push of a button, and this man is considered a sane and responsible person. a person that is called a defense secretary, one must question who on earth he is defending, his mouth from the words he has spoken, his honor (does he have any) or the people.
What more can one say, bring in the men with the white coats to take him away, clearly he is having a psychotic break with reality.
He's a happy puppy.
Pressing buttons - vs - skilled negotiation and conflict resolution.
Makes you you wonder what they learned in life.