BRUSSELS — European leaders on Thursday toughened their stance against a British proposal on how to structure their future relationship, with the European Council president saying Prime Minister Theresa May’s controversial “Chequers” plan “will not work.”

Mrs. May has cast that plan for Britain’s withdrawal from the European Union as the only workable one on the table.

But at an informal summit meeting in Salzburg, Austria, on Thursday, the council president, Donald Tusk, and fellow European leaders made clear that in their eyes, that proposal was dead.

If Mrs. May had hoped that the two-day Salzburg meeting would make her political life a little easier — her Conservative Party holds a conference at the end of September — it now appears it will have the reverse effect, and prompt more domestic criticism of her leadership.