"She should negotiate the best way she knows how, but it's too bad what's going on," Trump told The Sun.
Trump’s interview came after a contentious meeting in Brussels with NATO members.
Also in the interview, Trump slammed London Mayor Sadiq Khan, a Muslim, for his handling of terrorist attacks on the British capital.
"You have a mayor who has done a terrible job in London," Trump told The Sun, adding that Khan had "done a very bad job on terrorism."
The President also said that the anti-Trump protests about his trip made him feel unwelcome in London, the tabloid reported. Protesters flew a 20-foot blimp depicting Trump as an angry baby, CNN reported.
Sarah Wollaston, Conservative member of Parliament, said Trump was "determined to insult" May, The Guardian reported.
“The divisive, dog-whistle rhetoric in his Sun interview is repulsive,” Wollaston told the newspaper. “If signing up to the Trump world view is the price of a deal, it’s not worth paying.”
On the other side of the British political spectrum, Ben Bradshaw of Labour said courting Trump was “humiliating.”
"Our prime minister is so weak she still rolls out the red carpet for a man who does nothing but insult her. Humiliating," Bradshaw told The Guardian.
A giant balloon of Trump clad in a diaper in a 20-foot air-filled balloon went airborne Friday morning over Parliament, beginning a day of protests, CNN reported.
The orange-colored "Trump baby" blimp was inflated in Parliament Square at about 9.30 a.m. and rose about 100 feet in the air. It was grounded after an hour, CNN reported.