The Ministry of Defence has been condemned for a “dismal” failure to dispose of decommissioned nuclear-powered submarines.
The MoD has submarines which have been in storage longer than they have been in service and the UK now has twice as many submarines in storage as it does in service.
The National Audit Office (NAO) said the department has not disposed of any of the 20 boats no longer in service since 1980.
Some of these vessels still contain nuclear fuel and the failure to address the issue risks damaging the UK's international reputation as a "responsible nuclear power".
The issue was raised during Prime Minister's Questions by Labour MP Luke Pollard who asked whether the prime minister will extend the nuclear clean up to include all the royal navy submarines.
Mrs May responded to say the MoD will continue to work with the nuclear decommissioning service to achieve "steady state disposal of our laid up submarines."
The estimated cost of disposing of a submarine is £96 million, the NAO said.
Decommissioned vessels are being stored at Devonport and Rosyth, while arrangements are made to safely dispose of them and the radioactive waste they contain.
No submarines have been defuelled since 2004, when regulators said facilities did not meet required standards.
The process is not due to start again until 2023 and has been delayed for 11 years, with a £100 million cost increase to £275 million, a £12 million annual bill for maintaining and storing the nine fuelled submarines and pressure on dock space at Devonport.
The MoD has put its total future liability for maintaining and disposing of the 20 stored and 10 in-service nuclear-powered boats at £7.5 billion over the next 120 years, underlining the long-term nature of nuclear waste.
The Government said the ministry "needs to get a grip urgently" on the matter.
Meg Hillier, chairwoman of the Commons Public Accounts Committee, said: “For more than 20 years the Ministry of Defence has been promising to dismantle its out-of-service nuclear submarines and told my committee last year that it would now address this dismal lack of progress.
“It has still not disposed of any of the 20 submarines decommissioned since 1980 and does not yet know fully how to do it.
“The disposal programmes have been beset by lengthy delays and spiralling costs, with taxpayers footing the bill.
“The ministry needs to get a grip urgently before we run out of space to store and maintain submarines and we damage our reputation as a responsible nuclear power.”
The vessels being stored include the first submarines used to carry the UK’s nuclear deterrent – the Polaris boats HMS Revenge, HMS Renown, HMS Repulse and HMS Resolution.
Attack submarine HMS Conqueror, which sank the Argentine cruiser General Belgrano during the Falklands War is another of the boats in storage.
A Ministry of Defence spokesman said: ““The disposal of nuclear submarines is a complex and challenging undertaking.
“We remain committed to the safe, secure and cost-effective de-fuelling and dismantling of all decommissioned nuclear submarines as soon as practically possible.”