Jeremy Corbyn has undermined national security by vowing to never use nuclear weapons if he ever becomes prime minister, David Cameron has said.
The Labour leader sparked a fierce row in the final hours of Labour's party conference in Brighton by telling journalists he would never deploy weapons of mass destruction.
Speaking on BBC One's The Andrew Marr Show, Mr Cameron said: "The problem with his answer is 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.
"If you give any other answer then you are, frankly, undermining our national security, undermining our deterrent."
Mr Corbyn was asked by BBC Radio 4's Today programme if he would use nuclear weapons as prime minister.
He said: "No, 187 countries don't feel the need to have a nuclear weapon to protect their security, why should those five need it themselves? We are not in the era of the Cold War any more."
He added: "I am opposed to the use of nuclear weapons, I am opposed to the holding of nuclear weapons. I want to see a nuclear-free world. I believe it is possible."
Mr Corbyn's remarks provoked an explosive row in his shadow cabinet, with shadow defence secretary Maria Eagle insisting they were "unhelpful".