Theresa May has "full faith" in the UK ambassador who criticised the Trump administration in leaked emails but she does not agree with his assessment.
The PM's spokesman added that the leak was "absolutely unacceptable" and No 10 had made contact with the White House.
Sir Kim Darroch, the UK's ambassador in Washington, described Donald Trump's administration as "inept" in emails.
A senior Conservative MP has asked the Metropolitan Police to open a criminal investigation into the leak.
The US president responded to Sir Kim's comments by saying "we're not big fans of that man and he has not served the UK well".
The prime minister's spokesman said it was "the job of ambassadors to provide honest and unvarnished opinions" but Mrs May "does not agree with the assessment".
"The leak is absolutely unacceptable and, as you would expect, contact has been made with the Trump administration setting out our view that we believe that it is unacceptable," he added.
The chairman of the foreign affairs committee, Tom Tugendhat, told the House of Commons on Monday he had written to Met Commissioner Cressida Dick to ask for a criminal investigation.
"I have asked her for reassurance that all necessary resources will be made available to ensure that the source of this leak is determined as a priority," he said.
Foreign Office minister Sir Alan Duncan responded by saying a cross-government investigation would be "comprehensive" and the police "could be involved" if there was evidence of criminality.
Trade secretary Liam Fox told the BBC the leak was "unprofessional, unethical and unpatriotic", adding that whoever released the emails had "maliciously" undermined the defence and security relationship with the US.
"I hope if we can identify the individual, either the full force of internal discipline - or if necessary the law - will be brought to bear because this sort of behaviour has no place in public life," he said.
But Labour's shadow foreign secretary Emily Thornberry said Sir Kim had been "betrayed" and "hung out to dry even though his only crime was to tell the truth".
She added: "He told the truth about Donald Trump and that was because it was his job."
Analysis: Damage is 'considerable'
By James Robbins, BBC diplomatic correspondent
Just imagine if every heavily encrypted report to Whitehall from all UK ambassadors overseas was instantly available on your mobile.
The candour would cease immediately and they'd become ultra-bland and useless as a tool in policy-making.
So, damage in this case is considerable. There will be a large number of potential suspects.
Diplomatic telegrams are seen by scores, often hundreds of people - ministers and officials - across several departments. That is to ensure grown-up and private conversations can be had based on large amounts of source material.
Of course, there is damage to relations between the UK and the Trump White House too.
Mr Trump likes to dish out insults and criticism (remember his frequent belittling of Theresa May over Brexit, and his all out verbal attacks on the mayor of London) but he is pretty thin-skinned when the verbal arrows are aimed at him.
The one person who is not under suspicion in London is Sir Kim himself. After all, as his current political master, Mr Hunt, has made clear, he was just doing his job.
As the Foreign Office launched an investigation into the source of the leak to the Mail on Sunday, Mr Trump told reporters in New Jersey: "We're not big fans of that man and he has not served the UK well.
"So I can understand it and I can say things about him but I won't bother."
In the emails, the UK ambassador to Washington said: "We don't really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction-riven; less diplomatically clumsy and inept."
Sir Kim questioned whether this White House "will ever look competent" but also warned the US president should not be written off.
Dating from 2017 to the present day, the leaked emails said rumours of "infighting and chaos" in the White House were mostly true and policy on sensitive issues such as Iran was "incoherent, chaotic".
Although the Mueller investigation later found allegations of conspiracy between the Trump campaign and Russia were not proven, Sir Kim's emails said "the worst cannot be ruled out".
Speaking to BBC Radio 4's Today programme, Brexit Party leader Nigel Farage said if Boris Johnson becomes prime minister and the UK leaves the EU by 31 October, "people like" Sir Kim would "not be around".
Asked about speculation that he might take on the diplomatic role, Mr Farage said: "I don't think I'm the right man for the job", adding that he was "not a diplomat".
However, he said he "could be very useful" when dealing with the US administration.
Who is Sir Kim Darroch?
Sir Kim is the British ambassador to the US, which means he represents the Queen and UK government interests in the US.
Born in South Stanley, County Durham in 1954, he attended Durham University where he read zoology.
During a 42-year diplomatic career, he has specialised in national security issues and European Union policy.
In 2007, Sir Kim served in Brussels as UK permanent representative to the EU.
He was the prime minister's national security adviser between 2012 and 2015, dealing with issues such as the rise of the Islamic State group in Iraq and Syria, Russian annexation of Crimea, the nuclear threat from Iran and the collapse of government authority in Libya.
He became ambassador to the US in January 2016, a year before Donald Trump's presidential inauguration.