In a brief exchange during Prime Minister’s Questions last month, the leader of the Liberal Democrats, Ed Davey, asked Keir Starmer about Trident replacement. ‘We have to make a choice now,’ Davey said: ‘lease new missiles from the United States, accepting whatever terms the president gives us, or build our own here in the United Kingdom.’ The prime minister replied that Davey was ‘advocating a plan without knowing how much it would cost and how it would work’. The discussion moved on.

Both men spoke of Britain’s ‘independent nuclear deterrent’. But the UK’s nuclear weapons capability is dependent on the US. Not only does Britain rent its Trident missiles from America, but the British-built warhead designed to be carried by those missiles, the Holbrook, is closely based on the American W76. The Los Alamos National Laboratory announced last year that a replacement for the W76 is going ahead: the W93 should be ready by 2034.

There is no need for the UK to replace its warheads.…