A quarrel over visa-free travel to the US threatened on Tuesday to detonate a furious row at a European Union summit later this week and expose the bloc’s disunity in its dealings with the world’s great powers.

“The atmosphere is extremely tense on this. It might explode in a really bad way,” said one EU ambassador, speaking ahead of the Brussels summit, which will be attended by the bloc’s 27 heads of state and government on Wednesday and Friday.