LONDON — As Prime Minister Theresa May of Britain continued her effort on Monday to sell her agreement to withdraw from the European Union to skeptics in Parliament, she was blindsided by President Trump, who called the agreement “a great deal for the E.U.,” and warned that, if it were adopted, it might obstruct a free trade deal between the United States and Britain.

Mrs. May has long held out the prospects of a new trade deal between the United States and Britain as one of the payoffs of leaving the bloc, and negotiations were expected to begin as early as next spring, after Britain’s March 29 exit, known as Brexit.

According to Mrs. May’s proposed withdrawal agreement, a bilateral trade deal could come into force at the end of a transition period in 2020.

But Mr. Trump’s comments suggested that Britain might never be able to conclude a bilateral trade deal with the United States, creating new difficulties for Mrs. May. She must overcome the resistance of around 90 lawmakers within her own party if she is to win parliamentary approval for the agreement next month.