The US Ambassador to Panama John Feeley, a career diplomat and former Marine Corps helicopter pilot, has resigned, telling the State Department he no longer feels able to serve President Donald Trump.

"As a junior foreign service officer, I signed an oath to serve faithfully the president and his administration in an apolitical fashion, even when I might not agree with certain policies. My instructors made clear that if I believed I could not do that, I would be honour bound to resign. That time has come," Feeley said, according to an excerpt of his resignation letter read to Reuters.

A State Department spokeswoman confirmed Feeley's departure, saying that he "has informed the White House, the Department of State, and the Government of Panama of his decision to retire for personal reasons, as of March 9 of this year."

Meanwhile President Donald Trump has cancelled a trip to London scheduled for next month to open a new embassy, saying he did not want to endorse what he understood was an Obama-era decision to move out of the old one.

The cancellation is a further blow to relations between the allies. More than a year into his presidency, Trump has yet to visit London, with many Britons vowing to protest against a man they see as crude, volatile and opposed to their values on a range of issues.

"(The) reason I canceled (sic) my trip to London is that I am not a big fan of the Obama Administration having sold perhaps the best located and finest embassy in London for 'peanuts,' only to build a new one in an off location for 1.2 billion dollars ($A1.7 billion)," Trump said in a tweet late on Thursday.