MPs have demanded a vote on the use of unmanned drones after David Cameron unveiled plans to double their number and said he could envisage using nuclear weapons.

Conservative Defence Committee chairman Julian Lewis called for a Commons vote if the new drones were going to kill people in Syria.

Ministers said they would replace the UK's 10 drones with twice as many of a new, improved design in a bid to tackle the threat from Isis.

The announcement comes just weeks after Mr Cameron announced an RAF-operate drone killed two Britons in Syria.

The Tory leader also insisted he would be prepared to deploy the UK’s nuclear deterrent.

There were circumstances in which it would be "justifiable", he said.

He also hit out at Labour leader Jeremy Corbyn, accusing him of "undermining our national security" by saying he would never press the nuclear button.

The Tories have made defence and security one of the main themes of their conference, in a bid to capitalise on what they see as one of Labour’s key weaknesses.

Mr Cameron also denounced Russian leader Vladimir Putin for a "terrible mistake" in backing the Syrian leader, who he described as the "butcher Assad".

Russian support had made the region more unstable, he said.

Meanwhile, the former head of the Navy, Lord West of Spithead, has reportedly threatened to quit Labour if the party adopts Mr Corbyn's policy on unilateral disarmament.

But the leader of the Unite union, which backs Trident renewal, suggested that the Labour row over nuclear weapons was pointless.

The order to replace the nuclear deterrent will “get through” a crunch Commons vote next year, he predicted, no matter how Labour MPs vote because the Conservatives have an absolute majority.

Meanwhile, Defence Secretary Michael Fallon told Tory activists that with Isis Britain faces its "most serious threat in a generation", in what was seen as a signal ministers are moving closer to a vote on bombing inside Syria.

On the Trident nuclear deterrent, 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 Cameron also reiterated that he would not order RAF airstrikes in Syria until after a vote by MPs.

"Obviously, we are not going to do that until we have a vote in the House of Commons," he said.

Foreign Secretary Philip Hammond said there is "no place to hide" for Isis, adding Britain will defeat the terror groups "vile ideology, no matter how long it takes".