Donald Trump could threaten to pull US troops out of Ukraine and refuse to take part in joint Nato exercises if Britain and other European countries fail to commit to increased spending on defence, military figures and diplomats fear.

Senior sources involved in preparations for this week's Nato summit are concerned that the US president will begin talks with Vladimir Putin about "redrawing the security landscape" across Europe if leaders refuse to meet Mr Trump's demands to shoulder a bigger share of the military "burden".

Military and diplomatic figures told The Telegraph of fears over a possible offer that Mr Trump could make to Mr Putin at a planned summit between the two leaders in Helsinki on July 16 - four days after Thursday's summit in Brussels.

Separately, speaking to The Telegraph, Tobias Ellwood, the defence minister, warned: “Given the US’s footprint in Europe and Putin’s recent hostile actions that has eroded continental stability, much hinges on the outcome of the two forthcoming summits. And Britain, as Europe’s largest and most capable military force, is right to play an influencing role in US thinking.”

It comes as Britain’s ambassador to America today launches a staunch defence of the UK-US relationship under Donald Trump, insisting military ties remain "stronger than ever" despite some policy differences.