Sir Keir Starmer has been told to put plans in place for a "terrifying strategic betrayal" of Donald Trump bringing the UK-US Trident nuclear missile partnership to an end.
The missiles, designed in the US by Lockheed Martin, are jointly maintained - which costs far less than if the Royal Navy were to carry out the work on its own.
But much of Britain's nuclear stockpile is kept in the States, while in-service Trident missiles are sent across the Atlantic for periodic refurbishing.
Defence industry analyst Nicholas Drummond said that while it was "extremely unlikely" that Trump would sever the nuclear partnership, Britain needed to plan for a future in which he refuses to give the UK the missiles for its nuclear deterrent.
Much of Britain's nuclear stockpile is kept in the States, while in-service Trident missiles are sent across the Atlantic for periodic refurbishing
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Such a refusal would be a "strategic betrayal on a grand scale that would damage him and America," Drummond told The Telegraph.
"When it comes to support and maintenance, I would say that we are largely dependent on the US for parts and technical assistance. If this was withdrawn, it would also weaken our deterrent," he said.
"Anyone who suggested this a year ago would have been dismissed as an idiot. Now it is a scenario that we need to plan for."
Former Defence Select Committee Chair Tobias Ellwood, however, counselled calm.
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'Anyone who suggested this a year ago would have been dismissed as an idiot. Now it is a scenario that we need to plan for,' Drummond said
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"Whatever [Trump] decides, it won't affect our current ability - because he will be out of office by the time the missiles need to be replaced," he said.
Britain had originally purchased 58 missiles as part of a stockpile at King's Bay in Georgia - the "home base" of several US Ohio-class submarines which use the Trident weaponry.
By 2008, there were 50 left after test firings, research by the University of Bradford estimates.
The UK and US are still both working together on a new nuclear warhead, the W93, intended for use on American submarines by 2040.
Rusi's Matthew Savill said Britain could be forced to look to France for help
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Matthew Savill, director of military sciences at leading defence think tank the Royal United Services Institute (Rusi), said Britain made a huge saving by buying the missiles from a joint US-UK stockpile.
"If the US cut off support we would have a load of Trident missiles [in reserve] but at some point we would need to fashion our own missiles with somebody," he said - and suggested Britain could look across the English Channel for help from Emmanuel Macron's "umbrella".
"We could in due course replace these things - but the cost would be excruciating.
"They are the most terrifying weapons known to humanity... But the Russians have thousands and the Americans have thousands. We and the French have a few hundred. Is that enough to deter? We are entering uncharted territory."
