LONDON — Like ex-spouses who have just signed divorce papers but not yet begun the ugly business of dividing their possessions, Britain and the European Union are tiptoeing around each other as the year begins, desperate to keep their dealings civil even as they gird for the unpleasantness to come.

On Wednesday, a cordial Prime Minister Boris Johnson welcomed the new president of the European Commission, Ursula von der Leyen, to 10 Downing Street but offered no hint of flexibility on his campaign to negotiate a trade deal by the end of 2020.

Dr. von der Leyen spoke warmly of a new beginning for “old friends,” but she made it clear she thought Mr. Johnson’s timetable for negotiating a comprehensive agreement was utterly unrealistic. She warned Britain that “the more divergence there is” between Britain’s rules and the bloc’s, “the more distant the partnership has to be.”

“The truth is that our partnership cannot and will not be the same as before,” Dr. von der Leyen, a former German defense minister, said in a speech at the London School of Economics. “It cannot and will not be as close as before, because with every choice comes a consequence.”