Theresa May said the language used to refer to the women was completely unacceptable. (FILE PHOTO)

Outgoing British Prime Minister Theresa May on Monday condemned US President Donald Trump's tweets telling progressive Democrat congresswomen to "go back" where they came from as "completely unacceptable".

"Her view is that the language which was used to refer to the women was completely unacceptable," May's spokesman told reporters.

Trump and May have endured a rocky relationship, which took a turn for the worse last week following the leak of diplomatic British cables highly critical of his presidency.

Angered by May's support for her ambassador, Trump assailed the prime minister over her handling of fraught Brexit negotiations and welcomed her impending departure from office.

"What a mess she and her representatives have created," Trump fumed in a series of tweets. "I told her how it should be done, but she decided to go another way."

"The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister," he wrote.

Boris Johnson is the heavy favourite to replace May as prime minister and has spoken out in support of the US president, with both men enthusiastic backers of Brexit.

Johnson wants to visit Trump immediately when he becomes prime minister to try to negotiate a post-Brexit trade deal, according to the Times on Monday.

But May's comments will now raise the pressure on him to condemn the president's comments.

Trump accused congresswomen who "originally came from countries whose governments are a complete and total catastrophe" of "viciously telling the people of the United States, the greatest and most powerful Nation on earth, how our government is to be run."

"Why don't they go back and help fix the totally broken and crime infested places from which they came. Then come back and show us how it is done," Trump wrote.