The US embassy in Britain confirmed Monday the resignation of ambassador William Farish, saying he would leave his post next month after less than three years in the job.
The embassy told AFP the resignation would be "effective July 10".
It would not say who would replace Farish, a Texas businessman and Bush appointee, or whether the appointment would take place before the US presidential election in November.
In an editorial published Saturday in the Times newspaper, Farish said it was time to "turn my attention to family and business".
But he added that relations between the United States and Britain had never been closer than during his tenure, since July 2001.
"There is no time that the 'special relationship' has been better", he wrote, brushing aside any protests by saying that "history often renders a different judgement from contemporary boos and catcalls".
London and Washington together have earned the "justifiable pride" of their citizens with the intervention in Afghanistan to depose the Taleban regime and the Iraq war that ousted Saddam Hussein, he continued.