Theresa May has struggled in Brexit negotiations because she lacks “the basic human skills to be a political leader”, an EU figure involved in exit negotiations has said.

In a remarkable assessment that illustrates EU frustration at the prime minister, Philippe Lamberts, a Belgian MEP who sits on the EU legislature’s Brexit steering group, said she had left European leaders “upset” time and time again with her robotic approach.

“If there’s one lesson we have learned from Theresa May’s attitude is her inability to factor in what others think, her inability to form bonds of trust within her Cabinet,” he told reporters in Strasbourg.

“Again we’ve seen it between her Cabinet and the House of Commons but also within the European Council.”

Citing Xavier Bettel, the Luxembourg prime minister’s reception to the prime minster’s address on Friday, he added: “When you listen to Xavier Bettel – Mr Nice Guy by definition – Xavier Bettel is someone who always tries to look at the good side of things.

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“When he exited the Council, you feel like if even Xavier Bettel is upset by Theresa May she must be totally devoid of the basic human skills you need to be a political leader. And that is scary. That is scary.”

Throughout Brexit talks the prime minister gone into meetings with EU leaders at summits and left with things worse rather than better – most notoriously in Salzburg, where leaders hardened their stances after hearing from the prime minister.

She repeated the trick at the European Council in Brussels and December, and was again said to leave leaders unconvinced at a meeting in the EU capital last week.

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Diplomats in the room at summits have throughout negotiations have said the prime minister’s addresses to leaders often resemble her reading out a statement, and that she often appears “evasive” response to questions.