Brussels braces itself for 'tough' Theresa May

"She is pragmatic," one official said on condition of anonymity, because of the sensitivity of the issue. "She is somebody we can work with," the source added.

Knowing her from council meetings of home affairs ministers, another EU official said: "She is tough. She is very eloquent, and she stands her ground."

Officials in Brussels say that even though the UK has opt outs from many of the policies on EU home and justice affairs, May has attended all the meetings and was very active.

She pushed hard for security cooperation and for member states to be able to collect passenger name records from intra-EU-flights.

One source said she coined the now mantra-style phrase in migration policy: "Breaking the business cycle of the people smugglers."

Other sources tell in chorus that May is rational, direct, straight to the point, serious and cold. They agree that she will be a hard negotiating partner.

"She was well-briefed, and felt comfortable at the meetings," one source said, who added that it won her appreciation around the table.

The EU official added that even when the UK was not involved in a policy, she intervened, trying to help forge a consensus.

"She behaved as part of the family," said the source.

There is also a sense of relief that the new British prime minister will be someone who knows how the EU works and would need to spend little time to assess what can be achieved, and what is impossible.

"She knows the EU institutions, the context, the backrooms. She is aware of all the complexities and that it is an advantage for both Britain and the EU-27 in the upcoming negotiations," an EU diplomat said, admitting that because of her knowledge of the Union's workings, May was a "preferred candidate" in the EU for the Tory leadership.

'Weak spot'

Already on Tuesday there was no explicit call from the European Commission for May to launch the Article 50 procedure as soon as possible.

Commission president Jean-Claude Juncker will send Theresa May a letter on Wednesday.

Sources said the letter will be standard for incoming prime ministers of member states. "It will only be congratulation," a source said, adding there will be no invitation or call to trigger the Article 50 exit procedure.

When asked by a journalist whether the commission chief can handle May, whom Tory MP Ken Clarke earlier labelled a "bloody difficult woman", the commission's chief spokesperson Margaritis Schinas quipped: "I am sure Jean-Claude Juncker can cope."

May is also known in Brussels for what a diplomat described as her "colorful, interesting" shoes. The soon-to-be British PM herself said her passion for eccentric shoes was her "weak spot", the diplomat added.