Britain must provide more clarity on its Brexit position in order to "start negotiating seriously", the EU's chief negotiator has warned.
As the third round of talks continued in Brussels, Michel Barnier said he was concerned that "time is passing quickly", while noting the lack of progress made after almost five months of a two-year negotiation.
“I am ready to intensify negotiations over the coming weeks in order to advance", Mr Barnier told reporters, before stating: "we need UK positions on all separation issues."
British negotiators are keen to move discussions to the future relationship with the bloc, but Brussels wants more clarity on the terms of the divorce first.
Britain has so far refused to explain how it thinks its liabilities to the EU should be calculated, let alone put a figure on it, which has infuriated Brussels, considering the figure is estimated to be between £50 million and £80 million.
Brexit minister David Davis hit back at criticism which suggested a lack of clarity on Britain's Brexit position insisting the UK position papers were strong in detail.
He said: "They are the products of hard work and detailed thinking that has been going on behind the scenes not just the last few weeks, but for the last 12 months, and should form the basis of what I hope will be a constructive week of talks."
He added that Britain wants to "lock in the points where we agree, unpick the areas where we disagree, and make further progress on a range of issues" and highlighted that would "require flexibility and imagination from both sides."
In recent weeks, Britain has published a series of position papers, which Mr Barnier said EU officials had read "very carefully", but said positions were needed on "all separation issues".
"We must start negotiating seriously," Mr Barnier said. "We need UK papers that are clear in order to have constructive negotiations, and the sooner we remove the ambiguity, the sooner we will be in a position to discuss the future relationship and a transitional period."
As things stand, Britain has little idea what relationship it will have with the other 27 countries of the EU past the Brexit date of March 2019 but attempts are already being made to shore up trade deals between the UK and other non-EU countries.
The third round of talks came amid separate reports that a visit to Japan by Prime Minister Theresa May this week could lead to disappointment over hopes for early trade deal talks with Tokyo.