Chairman of The Brexit Party Richard Tice MEP has warned that Prime Minister Theresa May must be not allowed to appoint the next British ambassador to the United States.

“Theresa May must not appoint the new U.S. ambassador. It is vital it is done by a new prime minister to reset relations with White House,” Mr Tice wrote on Thursday, adding that Sir Kim Darroch’s replacement should be a “pro-Brexit businessperson who can accelerate free trade talks”.

Sir Kim resigned his post on Wednesday following leaks of 2017 diplomatic cables where the ambassador called President Donald J Trump “insecure” and his administration “inept”.

The warning from the Brexit Party MEP came as Tory ministers have reportedly urged Prime Minister May to replace the ambassador now, before the appointment of her successor at the end of this month.

“They think it is in the national interest that we have an ambassador in place,” a source told The Times.

Doing so would deny Tory Party leadership candidate frontrunner Boris Johnson a chance to appoint his own pro-Brexit diplomat, leaving it in part to his rival, foreign secretary Jeremy Hunt.

Mr Johnson’s allies have warned Mrs May not to make a move on the appointment, with one senior ally telling the newspaper: “With two weeks to go before the new prime minister takes over it would seem rather odd if she plunged in and appointed someone. It’s such an important decision you would think it would be made by the new prime minister.”

On the BBC’s Newsnight on Wednesday, Mr Tice had said that after the leak, Sir Kim’s position was “completely untenable” and that he would have had “no credibility” with the White House had he stayed on.

“It’s a huge opportunity now for the new prime minister — not for Theresa May, I’m appalled by that Times suggestion — for a new prime minister to completely reset the agenda in a pro-Brexit world,” he said.

Leading Conservative Eurosceptic Steve Baker MP also backs appointing a diplomat who can make a “strong, heartfelt case for our future status as an independent trading nation” and who can work with the U.S. Congress, telling The Times that “a new ambassador must be appointed by the new prime minister. It would be totally unacceptable in such circumstances to tie the future prime minister’s hands.”