LONDON — After two apparently unproductive days in Northern Ireland, Prime Minister Theresa May was headed to Brussels for further talks on Thursday, hoping to put her Brexit plans back on course. Any hopes that she would receive a warm welcome, however, were dashed before she could board the plane.

On Wednesday, in a breach of diplomatic protocol, the president of the European Council, Donald Tusk, vented his frustrations with the endless political machinations in London with a stinging indictment of the most fervent proponents of Britain’s withdrawal from the European Union, or Brexit.

“I’ve been wondering what that special place in hell looks like, for those who promoted Brexit, without even a sketch of a plan how to carry it out safely,” Mr. Tusk said at a joint news conference in Brussels with the Irish prime minister, Leo Varadkar.