TAXPAYERS face a bill of up to £500 million following delays in a refurbishment of a nuclear submarine, it has emerged.

The Trident missile-armed HMS Vanguard was supposed to return to the Faslane naval base on the Clyde in 2018 after a three-year-long refuelling and refit but reports today say the Ministry of Defence has confirmed it is still being worked on in a dockyard in England.

Defence sources told the Sunday Times the £3.75 billion submarine is not expected to rejoin the fleet until the middle of next year at the earliest — four years late.

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The delays raise fears the public purse will have to pick up a bill topping half a billion pounds which is more than twice the original estimate for the work. 

SNP MP Martin Docherty-Hughes said the revelations underline that Trident is "not fit for purpose" and should be axed.

"These latest revelations show just how unfit for purpose the UK's nuclear deterrent actually is," said Docherty-Hughes, who sits on the Commons Defence Select Committee. 

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"Successive Westminster governments have obsessed over outdated and immoral nuclear weapons - with soaring costs and sinking military relevance - rather than fixing the multi-billion pound black hole in the conventional defence budget.

"The UK government needs to wake up to the reality in front of it and admit it's time to consign nuclear weapons to the dustbin of history."

The Royal Navy operates a cycle or rota of patrols by the four Trident-armed submarines — depicted in the recent BBC television drama Vigil — to ensure that one is always at sea and ready to launch its missiles. A smooth switching between patrols and maintenance periods is considered vital.

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Tobias Ellwood, the Conservative chairman of the Westminster defence committee, said a seven-year refit effectively means the navy has been running the deterrent on three submarines. 

“If something happens to your third submarine, you end up oscillating patrols between just two submarines and that puts your deterrent ability at risk,” he said, adding this made the deterrent vulnerable to an interruption of the supply chain to the Faslane naval base. 

“This needs to be fixed and HMS Vanguard returned to sea,” Ellwood said. 

A combination of Covid-19 working restrictions and technical problems are blamed for the delays. 

Microscopic cracks were detected in fuel rods similar to those used on HMS Vanguard during trials at a test reactor at Dounreay in Caithness a decade ago.

This prompted the MoD to order HMS Vanguard to be refuelled even though its reactor was not designed to be refuelled for a second time.

Industry sources say this led to the delays by engineers from Babcock, which runs the dockyard at Devonport for the navy, as they tried to work out how to safely reopen and then close the reactor.

The company has the exclusive contract to refuel and refit all the Royal Navy nuclear submarines at Britain’s single submarine refitting dock at Devonport so the delays to HMS Vanguard have affected the plans to overhaul another Trident-armed submarine, HMS Victorious.

It urgently needs repairs and will have to be moved into the Devonport dry dock as soon as work on HMS Vanguard is finished.

The cost of refuelling HMS Vanguard was originally announced as £200 million but industry sources said the deal was not a fixed-price contract and it is now costing more than £75 million a year, potentially pushing the bill to more than £500 million.

The Royal Navy told the Sunday Times: “We are committed to working closely with Babcock to safely deliver the HMS Vanguard planned period of deep maintenance and refuel work. It would be inappropriate to comment on costs for commercially sensitive reasons.” 

Babcock declined to comment.