British Prime Minister Theresa May said Tuesday Britain's ambassador to the United States has her full support and she will not fire him.

It follows a statement by the US President Donald Trump that he would no longer work with Ambassador Kim Darroch after critical messages he sent to London were leaked.

A major investigation is under way to track down the mole, believed to be a Whitehall insider, who leaked the private messages Darroch sent to London in which he was highly critical of Trump.

Downing Street said at a media briefing Tuesday that May stood by the ambassador. A spokesperson for May described the leaked messages as "unfortunate" and added that Britain and the US still share "a special and enduring relationship".

The official spokesperson said: "We have made clear to the US how unfortunate this leak is. The selective extracts leaked do not reflect the closeness of, and the esteem in which we hold, the relationship."

The statement added that ambassadors need to be able to provide honest assessments of the politics in their country.

The Guardian newspaper in London said Tuesday that Downing Street reiterated that May would not be firing Darroch in her final two weeks as prime minister.

Darroch's highly sensitive message was intended to be read by a small number of people, including May and her senior ministers. It was never intended to be made public, and outlined the ambassador's personal view of Trump.

On Monday Downing Street said May did not agree with the ambassador's assessment but had full faith in him.

The mood of the diplomatic row escalated Monday night when Trump launched a scathing attack on May and said the U.S. government would no longer deal with the British ambassador to Washington.

The Guardian newspaper in London said Tuesday that Trump's latest statement creates a dilemma for the new prime minister, likely to be Boris Johnson, who will have to decide whether to replace Darroch and risk looking as if he has been bullied into the action by Trump.

"However, keeping him in post risks further damage to the UK's diplomatic relations with one of its closest international allies," said the Guardian.

Foreign Secretary Jeremy Hunt, who is also in the race to become Britain's next prime minister, said he did not agree with all the views expressed by Darroch, and insisted that Britain had the warmest of relationships with Washington.