Britain should have a completely independent nuclear deterrent as it can no longer rely on the US, Ed Davey is expected to say on Sunday.

In a speech at the Liberal Democrats spring conference, the party leader will argue that the UK should manufacture and maintain its nuclear weapons in Britain, a move that Davey acknowledges will cost billions.

Davey’s speech will come amid his claims that the US president, Donald Trump, has made his support for European security “conditional” on his personal whims.

“While Trump is in charge, we certainly cannot rely on America as a dependable ally in the way we used to,” Davey will say. “And we can no longer bet our nation’s security on the hope that the US won’t produce new versions of Trump in the future.

“So the real question is not whether we should build a sovereign British nuclear deterrent. The question is what happens if we don’t.”

In theory, a British prime minister could choose to launch nuclear missiles without input from allies, including the US.

However, the UK’s nuclear programme, Trident, based at Faslane, near Glasgow, on the River Clyde, is heavily dependent on US input. The weapons are manufactured in the US and have to be returned there regularly for maintenance.

Davey’s speech is likely to be viewed as the latest instalment of what has been dubbed Operation Epsom Fury – a play on Trump’s Iranian mission and an attempt to attract voters disillusioned by Britain’s relationship with the president ahead of May’s local elections.

“If the answer to ‘Is our nuclear deterrent working?’ depends on what Donald Trump had for breakfast, then the answer is, ‘No, it’s not’. And our deterrent is not truly independent,” Davey is expected to tell delegates in York.

“This should be keeping British defence planners awake at night. Yet it’s not being asked loudly enough in our public debate. Perhaps because Conservative- and Reform-supporting commentators don’t want to face up to the profound implications of Trump.

“Our nuclear deterrent – the ultimate guarantor of our national security, the thing that successive governments of every stripe have described as the bedrock of Britain’s defence – is not fully ours.

“The Trident missiles sitting in our Vanguard submarines are leased from the United States. Their maintenance depends on American facilities. And that means the operability of our deterrent ultimately depends on the goodwill of whoever sits in the Oval Office.

“A few years ago, that didn’t feel like an issue. It certainly feels like one now.”

Davey will cite Trump’s threat to annex Greenland and his apparent failure to take the Russian president, Vladimir Putin, to task over the war in Ukraine as evidence that he is an increasingly unreliable ally.

“He and his White House lackeys have made it clear, repeatedly and unmistakably, that American support for European security is conditional,” Davey will say. “Conditional on European countries doing what Trump wants, whether on trade, relations with China, or just being nice to him. Certainly nothing to do with the values and alliances that have kept us safe for 80 years.”

Davey is expected to concede that building a nuclear capability without US input would would “cost billions over the next two decades” at a time when defence spending is already under pressure.

However, he will add that instead of “handing billions of taxpayers’ hard-earned cash to the American defence and technology industry” the money should be invested in the UK.

“Let’s invest in British science and manufacturing, build up our defence industry, and guarantee a fully independent deterrent we can truly rely on, no matter who sits in the Oval Office,” he will say.

The Liberal Democrats have said they remain committed to the goal of multilateral nuclear disarmament.

However, Davey will say: “With Vladimir Putin sitting on a stockpile of more than 5,000 nuclear warheads, we must deal with the world as it is. Trump’s reckless, unpredictable presidency – and the reality that we can no longer count on America as we once assumed we could – is a challenge we cannot ignore.”