A Labour Party led by Jeremy Corbyn would pose a threat to national security by threatening the future of the UK's nuclear deterrent, George Osborne said.
The Chancellor said "an unholy alliance of Labour's left-wing insurgents and the Scottish nationalists" would shatter decades of near-unbroken Westminster consensus in favour of maintaining a capability.
Both Mr Corbyn, the frontrunner to succeed Ed Miliband, and the SNP are opposed to the renewal of the Trident missile system being pursued by the Conservative government.
But Mr Osborne, who is visiting the Faslane naval base, home of the nuclear submarine fleet, to announce a £500 million investment, said that would be "disastrous".
Amid suggestions that Conservatives were delighted at Mr Corbyn's surprise emergence as the favourite to lead the party, Mr Osborne insisted the contest must not be seen as a "a bit of a joke".
"On the contrary, I think we should take it deadly seriously," he wrote in The Sun.
"For the new unilateralists of British politics are a threat to our future national security and to our economic security. We're going to take on their dangerous arguments and defeat them."
The only breakdown in agreement over the need for a nuclear deterrent had being during the 1980s when Labour was dominated by the left, he said.
"Now that consensus, which is so important for our security and reliability as an ally, risks being shattered again by an unholy alliance of Labour's left-wing insurgents and the Scottish nationalists.
"This isn't an argument about the past - the return of the unilateralists to British politics threatens our nation's future security.
"In a world that's getting more dangerous it would be disastrous for Britain to throw away the ultimate insurance policy that keeps us free and safe."
His warning came as Andy Burnham claimed enough Labour backers are still to make up their minds to swing the result of the party leadership race in his favour.
The bulk of the 550,000-plus available votes are thought already to have been cast but Mr Burnham will tell a rally on Monday he is confident many of the undecided are switching back to his cause.
He will accuse "the Tory press" of falsely giving the impression the result is a foregone conclusion and claim he is the only one of Mr Corbyn's rivals to stand a chance of stopping him.
The other candidates are fellow shadow cabinet minister Yvette Cooper and Liz Kendall.
Tens of thousands of people are still to vote and there is everything to play for, he will say - though his camp said they did not have any details of the number of votes cast.
"The future of the Labour Party hangs in the balance. I believe I am the only candidate in this race that can stand in the way of Jeremy Corbyn.
"Over this long weekend, I have spoken to Jeremy Corbyn supporters who are now moving to my campaign. I feel encouraged going in to the final ten days and will be fighting for every vote.