Donald Trump reportedly berated Theresa May in a phone call while he was on Air Force One.
Mr Trump was on his way to Paris for events marking the centenary of the end of the First World War.
According to the report Mr Trump berated Mrs May, saying she had not been doing enough to rein in Iran.
He was also said to have questioned her over Brexit, and to have complained that US trade deals with European nations were unfair.
Mrs May’s aides were said to have been taken aback by the president’s tone, the Washington Post reported, citing officials briefed about the call.
Downing Street has yet to comment on the report. After he arrived in Paris, Mr Trump faced criticism for cancelling a scheduled visit to a US military cemetery because of bad weather.
Mrs May’s relationship with Mr Trump has been a rocky one. She was the first foreign leader to visit Mr Trump at the White House after he became president, but people close to Mr Trump suggested there was a lack of “chemistry” between the leaders.
On his visit to the UK in July Mr Trump embarrassed Mrs May, attacking her handling of Brexit in a newspaper interview, and saying “she didn’t listen to me”. However, he went on to apologise and lavish her with praise.