Britain's decision to spend £20bn on renewing its nuclear deterrent has been challenged by a group of retired senior military officers, who branded the Trident system "completely useless" against the threats of the modern world.

The former head of the armed forces, Field Marshal Lord Bramall, backed by two senior generals, argued in a letter to The Times that the nuclear deterrent is no longer truly independent and does not guarantee Britain a seat at the top table of international diplomacy in the United Nations Security Council.

The large sums being spent on replacing the Clyde-based submarines which carry the Trident missiles could be better used on conventional weapons, they suggested.

"Nuclear weapons have shown themselves to be completely useless as a deterrent to the threats and scale of violence we currently face or are likely to face, particularly international terrorism," the letter stated. "Our independent deterrent has become virtually irrelevant, except in the context of domestic politics."

General Lord Ramsbotham, who also signed the letter with General Sir Hugh Beach, said he wanted to restart the debate over the renewal of Trident, which was approved by the Commons in March 2007 He added: "We argue that it is conventional weapons we now need. Their pin-point accuracy, their ability to help our forces in the sort of conflicts that are taking place is something which means you have to question the huge expense of Trident."