Britain's nuclear deterrent is all but dead in the water, according to a nuclear expert, as the scale of the growing crisis caused by a possible defect in the nation's submarine fleet became apparent yesterday.

Three months ago the Sunday Herald was first to report that a problem with Royal Navy submarine HMS Tireless had thrown the safety of the rest of the Navy's fleet of Swiftsure class and Trafalgar class nuclear-powered submarines into doubt.

However, yesterday the Ministry of Defence admitted for the first time that checks were being carried out on the remainder of the fleet, following the accident on board Tireless in May which forced it to limp to dock in Gibraltar for repair.

The problem centred on a faulty weld in the sub's pressure circuit, part of the main nuclear reactor, which led to a coolant leak and the shutting down of the reactor. Senior MoD officials now concede it may have been caused by a generic design flaw.

The problem affects the fleet of 12 nuclear-powered hunter-killer submarines which carry conventional weapons. Most were berthed over the summer, but two - one Swiftsure-class and one Trafalgar-class - were at sea until yesterday, when they were recalled for checks.

The MoD insists the different design of the Vanguard class nuclear submarines, which carry Britain's entire Trident nuclear deterrent capability, means they are not affected by the crisis.

However, experts and opposition politicians last night warned that this is, at best, only partially true. At the very least, the loss of the hunter killers - which normally escort the Trident boats - will hugely curtail the operation of the nuclear deterrent.

John Large, a nuclear safety consultant retained by the Gibraltar government to advise it on the situation, said the withdrawal of the hunter-killer fleet left Trident all but redundant. "This is very serious. Of course it affects the Trident subs," he said. "Without the Swiftsure and Trafalgar fleets, the Navy can't support them. The MoD can't seriously plan to put Trident out without cover - they depend on having the hunter killers. It is an integrated flotilla, and this means that the nuclear deterrent is effectively moribund.

"This type of defect should have been detected some time ago if the monitoring programmes were right. The fact that it is a generic defect is very worrying and the situation as a whole is highly embarrassing."

Armed forces minister John Spellar last night denied that there were any confirmed problems with the other submarines: "We had very minor leaks on HMS Tireless, but there weren't leaks on other ships. But once we thought there was the possibility of a generic fault across the fleet in the cooling system we started to look at them. Once we found out there was probably a systematic problem, the only sensible and prudent thing was then to look at all the others."

He said a fuller examination of HMS Tireless had revealed that cracks in a pressurised pipe which had been thought to be only a few millimetres long were in fact 10 times as big. No timescale could be given for checks and possible repairs to the other submarines, he said.

Menzies Campbell, Liberal Democrat defence spokesman, said he would be seeking answers at a defence procurement debate at Westminster on Thursday. "This leaves a huge hole in Britain's defence capability," he said. "It is also a source of some embarrassment.

"MPs from all parties will be looking for some assurances from the government and a clear timetable as to when this gaping hole in Britain's defences will be plugged. The hunter-killer submarines are so integral to Britain's defence that if they are not operational there could be consequences far beyond their role of providing defence for the Tridents.

"The Trident submarines normally operate in tandem with the hunter-killers and they will now have to put to sea in less than ideal conditions," said Campbell. He added that the role of the hunter-killer submarines in deploying special forces on covert operations was another aspect of their operation that would be hard to replace. "MPs will want to know why it has taken so long to get to the bottom of this."

Shadow defence secretary Iain Duncan Smith blamed treasury-inspired cuts for the Navy's plight: "This is a desperate situation. The government has slashed the defence budget, and has not ordered a single warship since coming into power three years ago," he said.

He called on the government to reconsider the leasing of four Upholder class hunter-killer submarines to Canada. These submarines, built in the early 1990s, were mothballed in 1994 by the Conservative government as surplus to requirements. A final decision to lease them to Canada was taken by the current Labour government in 1998.

"We need to find some way in which we can hold on to the Upholders termporarily while this problem is sorted out," Duncan Smith said. "This is a major crisis and will curtail the range of operation of the Trident submarines, as well as hindering the ability of the special forces to take covert action. The government must move heaven and earth to get these ships repaired. This is a national priority."

However, a Navy spokesman ruled out the suggestion that the handover of the Upholder class submarines could be delayed. "The Upholders are to go to Canada in a lease-to-buy deal. One was delivered a fortnight ago and the other three will be handed over by the middle of next year," he said.

He also refused to confirm or deny reports that the same flaw which caused a coolant leak in Tireless had been found in six more submarines in the fleet. "Checks are still being carried out," he said. "The two submarines which are not here will join the others in undergoing checks.

"This is not a position which the Navy welcomes but it is one we are prepared to accept to ensure our untainted 40-year safety record remains unblemished."

He said the Trident submarines were unaffected, but insisted he could not discuss deployment issues.

"We wouldn't compromise the safety of the nuclear deterrent. It will continue to operate as it has done since 1962," he said.

John Ainslie of Scottish CND said submarines should have been taken out of service when the problem first emerged in May. "The MoD knew it was possible that this was more serious, but they knew it would mean taking Trident off patrol and that is the last thing they want to do."

Last night campaigners in Gibraltar insisted they would step up protests against HMS Tireless being repaired in the port there. "We have been totally vindicated," said Janet Howitt of the Concerned Parents group. "We will be pressing our government to reconsider permission for the repair."

However, a spokesman for chief minister Peter Caruana said: "This doesn't change the case for repairs in Gibraltar."

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