"UK will determine appointments based on our national interest alone," said Jeremy Hunt

Foreign minister Jeremy Hunt urged British diplomats to keep speaking "without fear or favour" following the resignation of the ambassador to the United States over leaked memos that criticised the U.S. administration.

Britain's ambassador to Washington, Kim Darroch, resigned on Wednesday after days of criticism from U.S. President Donald Trump following reports containing details of Darroch's diplomatic telegrams assessing the U.S. government.

The public spat has undermined the so-called special relationship between Britain and the United States at a time when the British government is looking to strengthen ties ahead of its departure from the European Union.

"Please keep speaking up without fear or favour, remembering that the UK government alone will determine appointments based on our national interest alone," Hunt said in a message to staff at the British foreign office.

An inquiry is underway into who leaked the memos, and why.

Hunt, who is the underdog candidate in a run-off to be the country's next prime minister, added: "I want you to know that you will always get all the support you need to carry out your vital work."

His rival leadership candidate, Boris Johnson, was criticised for refusing to back Darroch in a televised leadership debate on Tuesday night.