There are serious concerns they will miss a deadline for a divorce deal, and that the UK's departure is becoming more and more chaotic.
British Prime Minister Theresa May was bullish at parliament, saying she hoped her EU partners would make proposals at a new round of talks, opening the way to the next stage of negotiations.
"A new deep and special partnership between a sovereign United Kingdom and a strong and successful European Union, is our ambition and our offer to our European friends," Ms May said.
"Achieving that partnership will require leadership and flexibility, not just from us, but from our friends the 27 nations of the EU.
"As we look forward to the next stage, the ball is in their court, but I'm optimistic that it will receive a positive response."
Prime Minister May expects any deal with the EU to be struck close to the end of the two-year negotiating period, leaving Britain in the dark as to the costs or benefits until such a time.
"When we leave the European Union on the 29th March 2019, neither the UK nor the EU and its member states will be in a position to implement smoothly many of the detailed arrangements that will underpin this new relationship we seek," Ms May said.
Ms May has so far fought off attempts to unseat her by critics angry over an ill-judged election, which saw her lose her governing Conservatives' majority.
But her weakness has opened the door for many in her party to challenge her Brexit strategy, only 18 months before Britain plans to leave the EU.
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