Britain's nuclear submarines are "completely useless" against modern warfare, and the £20bn spent on renewing them is a waste of money, retired senior military officers said yesterday.

The former head of the armed forces Field Marshal Lord Bramall, backed by two senior generals, argued that the huge sums being spent on replacing the delapidated submarines that carry the Trident ballistic missiles could be better used to buy conventional weapons which are badly needed by the armed forces.

"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 group said in a letter to the Times. "Our independent deterrent has become ­virtually irrelevant, except in the context of domestic politics."

Retired army general Lord Ramsbotham, one of the signatories, said he wanted to restart the debate over the renewal of ­Trident, which was approved by the House of Commons in March 2007, despite a large rebellion by Labour MPs.

Ramsbotham told BBC2's Newsnight programme: "We argue that it is conventional weapons we now need. Their pinpoint 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, which is limiting what we can do."

Ramsbotham said he no longer believed that Britain's nuclear deterrent was truly independent.

"We don't own the missiles and it is absolutely unthinkable that we should ever consider using it or threatening to use it without having the clearance of the United States," he said.

"The fact is that Trident is an inappropriate weapons system. You can't see Trident being used against something like nuclear blackmail by international terrorism. It is a cold war weapon. It is not a weapon for the situation where we are now."

Ramsbotham said he was "quite certain" that some serving members of the military shared his doubts. He added that the government's decision to renew ­Trident was driven more by political considerations than by the true requirements of national defence.