LONDON — On Thursday, Prime Minister Boris Johnson appeared to have accomplished what his long-suffering predecessor, Theresa May, never could. He rallied support among lawmakers for a plan to extricate Britain from the European Union and won praise from some of the same legislators who had tormented Mrs. May.

One Conservative Party lawmaker even seemed to compare Mr. Johnson to Moses, the biblical figure who descended from the mountain with new commandments.

But diplomats in Brussels greeted his Brexit plan frostily and pointed out a series of gaps and problems. It was an ominous sign that, after more than three fraught and exasperating years of debate in Parliament, Brexit was heading once again for a deadlock — this time in Brussels.

European Union leaders have been polite, at best, about Mr. Johnson’s proposal. On Thursday, Donald Tusk, president of the European Council, said he had told Mr. Johnson he was “open but still unconvinced.”