Britain appointed its current U.N. envoy as the next ambassador to the U.S. on Friday after London's previous top diplomat in Washington resigned over comments about President Donald Trump.

Downing Street said Karen Pierce would replace Kim Darroch, who quit last year over leaked diplomatic cables in which he criticized Trump as "inept" and his White House administration "dysfunctional."

Relations between Britain and the U.S., which both sides often describe as a "special relationship," have fluctuated since Trump took office in 2017. He criticized Johnson's predecessor Theresa May's handling of Brexit, and there have been disagreements over Iran, plans for a tax on digital companies and most recently Britain's decision to allow Chinese firm Huawei a role in building its 5G mobile phone network.

However, both allies have said ties are strong, and U.S. Secretary of State Mike Pompeo said last week Britain would be "at the front of the line" for a post-Brexit trade deal with the U.S.

Pierce began her career at the Foreign Office in 1981 and was Britain's ambassador to Afghanistan between 2015 and 2016. She also served as permanent representative to the U.K. mission to the World Trade Organization among other international organizations.

"I am honored to have been asked to represent the U.K. in the U.S. I think it is the U.K.'s single most important relationship," she said in a statement.