“In order to achieve sufficient progress we need a serious British response.”

The implication, EU sources confirmed, is that the – in the opinion of the European Union – the UK is still not giving “serious” responses to European demands for a financial settlement and a deal on citizens’ rights to be hammered out before talks on a trade deal even begin.

Jean-Claude Juncker, the president of the European Commission, added his warnings about a “difficult” negotiation ahead.

He spelled out that Europe wants a detailed and legally water-tight deal guarantee EU citizens in the UK after Brexit, and would not settle for blanket political assurances from the UK side.

“This does not simply imply setting out a few principles,” he said.

“The negotiations will be difficult and it will be even be difficult to retain the unity we were able to construct today and we will do our utmost to retain that unity, which is important for the 27, but it is also important for the United Kingdom [to conclude a deal],” he added.

Mr Juncker also hinted at his frustrations with Mrs May at last week’s dinner after being chided by Mr Tusk for being too pessimistic about the prospects for a deal, urging his colleague to “be more optimistic”.