David Cameron has said there are circumstances in which he would launch a nuclear attack on another country.

The PM described nuclear bombs as “the ultimate insurance policy” and said the attack could be “justified”.

Mr Cameron’s statement comes after Labour leader Jeremy Corbyn said he would not use nuclear bombs on another country’s population.

“If you ... believe like me that Britain should keep the ultimate insurance policy of an independent nuclear deterrent, you have to accept there are circumstances in which its use would be justified,” Mr Cameron told BBC One’s Andrew Marr show on 4 October.

HMS Vanguard sits in dock at Faslane Submarine base on the river Clyde in Helensburgh, Scotland (Getty)

“If you give any other answer then you are, frankly, undermining our national security, undermining our deterrent.”

Mr Corbyn told BBC Radio 4’s Today programme last week that he would not use the weapons and that the UK should not be spending money on them.

“I don’t think we should be spending £100bn on renewing Trident. That is a quarter of our defence budget,” he said.

“There are many in the military that do not want Trident renewed because they see it as an obsolete thing thing they don’t need. They would much rather see it spent on conventional weapons.”

Prime ministers leave secret instructions for nuclear missile submarine captains to follow in the event of a nuclear wear. These orders are sealed in a letter only to be opened if the United Kingdom is destroyed.

These instructions can either be to launch a nuclear attack, to stand down, to assist a surviving allied power, or for the captain to use their judgement.

A nuclear strike would likely kill a minimum of tens of thousands of people if used. The last nuclear attack, on the Japanese city Nagasaki, killed around 80,000 people in a single blast.

Warheads carried on Britain’s nuclear submarines are eight times more powerful than the atomic bombs used in 1945.

Parliament is set to vote during this parliamentar on whether to renew the Trident nuclear weapons system.

Mr Corbyn opposes renewal, but some Labour MPs have said they disagree with him.

The Opposition’s official policy remains in support of Trident after the party’s leadership failed to secure a policy vote to change it at annual conference last week.