Joe Biden has done Britain a "bad turn" by blocking Ben Wallace's run for Nato chief, government sources have said.
The Defence Secretary had been the favourite with many member states to succeed Jens Stoltenberg, the current secretary-general, later this year.
But Mr Wallace admitted on Wednesday that his ambition to take over the reins, which he had repeatedly expressed in public, is now "not going to happen".
Allies of Mr Wallace have since told The Telegraph they believe his ambitions were thwarted after the US president refused to endorse his attempt to become the military alliance's next secretary-general.
Government sources have suggested there are now fears the snub may damage the special relationship.
It now seems likely that Mr Stoltenberg, the former Norwegian premier, will be asked to defer his retirement, originally set for later this year, by 12 months.
A government source said: "We're supposed to be their closest ally. And this is what we get.
"It is what it is . but it is bad for UK-US relations.
"They've done the UK a bad turn with their preference for this Danish lady."
The US president is said to have initially favoured Mette Frederiksen, the socialist prime minister of Denmark, who is being backed by France and Germany.
But one source said he cooled on her after she failed to impress him during a visit to Washington earlier this month.
The Nato secretary-general chairs meetings of the alliance's leaders, carries out diplomacy and oversees its army of 4,000 officials in Brussels.
Mr Wallace's candidacy had been popular with members, thanks to his part in the UK's high-profile role in stiffening Nato's resolve following the Russian invasion of Ukraine.
Boris Johnson, the former prime minister, and Rishi Sunak, the current occupant of No 10, had thrown their weight behind him.
However, he was facing an uphill struggle given that European leaders, led by Emmanuel Macron, the French president, are lobbying hard for the next boss to come from an EU state.
The Telegraph understands Mr Biden's decision not to endorse Mr Wallace's candidacy proved to be the final nail in the coffin and led to the Defence Secretary pulling out of the race.
Senior figures in Whitehall felt "badly let down" by the US decision, which they fear has sent out the wrong message to allies by snubbing a candidate from Britain.
The UK is one of only seven of the military bloc's 31 members to have met its target of spending two per cent of GDP on defence last year.
Of the 31 members, only the US, the UK, Greece, Poland and the Baltic states spend more than two per cent, but several major countries fall short. Last year Germany, France and Italy were all below the target figure.
A government source said: "We had better watch out about the future of Nato, as there might be more countries seeking to roll back on the two per cent commitment."
It is possible that the issue may also sour UK support for the US over other international issues, including Washington's trenchant position on Taiwan independence.
Whitehall sources suggested that support for the Danish premier fizzled out when "she failed the interview" on a visit to Washington.
A source close to Mr Wallace agreed with this assessment, commenting: "This is no time for amateurs."
By then however, the damage to Mr Wallace's candidacy had been done.
Instead of reverting to Mr Wallace, Mr Biden is thought to have next persuaded leading European allies, such as France and Germany, to agree to postpone a replacement decision for a year.
The Defence Secretary had backing in Eastern European countries, which appreciated Britain's support over Russian aggression, but was spurned by Paris and Berlin.
"Their view is that, if it's a European, it has to be someone in the EU so it doesn't fall foul of the Brexit arguments and EU's growing role in security," said one source.
"It's not just France but many nations. EU leaders, foreign ministers and defence ministers meet very regularly so not to be part of that, they believe, would be suboptimal."
A government source accused the French and German governments of "scrabbling about" to prevent Mr Wallace succeeding Mr Stoltenberg and suggested the UK had not received appropriate support from the US over the Nato post.
There was surprise when Antony Blinken, the US secretary of state, was recently asked if his country would be backing Mr Wallace.
"The United States isn't backing any candidate," he replied.
Mark Francois, a former Armed Forces minister, said: "Ben Wallace has been and still is an excellent Defence Secretary, as evidenced by his very capable response to the war in Ukraine.
"It would be a great shame if national politicking within Nato deprives the organisation of a potentially highly skilled secretary-general."
Washington is now trying to persuade Mr Stoltenberg, who has been in post for almost nine years and had planned to retire this summer, to stay on for another 12 months.
A senior European diplomat said that the move had broad support, given the desire to avoid a public spat between allies when "there's no consensus candidate".
Many allies want to appoint a woman to the top Nato job for the first time and believe that such a senior role should only go to a former head of government.
Kaja Kallas, the current Estonian prime minister, is favoured by some but is opposed by Paris and Berlin, who see her as too hawkish towards Russia.
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