Trident submarines will be replaced after Commons vote

Theresa May, making her first Commons speech since entering Number 10, said the “very real” threat posed by Russia and North Korea meant the UK could not afford to “relax our guard”.

She said a continuous at-sea deterrent from the four Vanguard class submarines was necessary and she would not hesitate in authorising a nuclear strike if it was needed to protect Britain.

She said: “We must continually convince any potential aggressors that the benefits of an attack on Britain are far outweighed by their consequences; and we cannot afford to relax our guard or rule out further shifts that would put our country in grave danger.

“We need to be prepared to deter threats to our lives and our livelihoods, and those of generations who are yet to be born.”

The issue divided the Labour Party whose MPS were given a free vote on the issue, despite leader Jeremy Corbyn voting against replacing Trident.

Whether to renew Britain’s nuclear deterrent programme has occupied Labour policy for decades with strong anti-nuclear defence sentiment within the party.

Labour leader Jeremy Corbyn reiterated his opposition to the potential use of the weapons - one of the key elements of the doctrine of nuclear deterrence.

He said: “I make it clear today that I would not take a decision that kills millions of innocent people. I do not believe the threat of mass murder is a legitimate way to go about dealing with international relations.”

He told MPs “we are not debating a nuclear deterrent but our continued possession of weapons of mass destruction”, which are capable of killing one million people per warhead.

He added: “What is the threat that we are facing that over a million people’s deaths actually deters?”

The head of the GMB trade union Tim Roache insisted Mr Corbyn should abide by existing Labour Party policy, which is to back Trident renewal. He warned that 45,000 jobs around the country - many of them highly skilled - were dependent on the programme going ahead.