The UK has been told by the EU's chief Brexit negotiator that it needs to take withdrawal talks "seriously".
Michel Barnier used the opening of a third round of talks in Brussels to insist that London must end "ambiguity" regarding key positions like the Brexit divorce bill.
He said: "To be honest, I'm concerned, time passes quickly. We need UK positions on all separation issues. This is necessary to make sufficient progress. We must start negotiating seriously.
"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.
"I am ready to intensify negotiations over the coming weeks in order to advance."
In response, the UK's Brexit Secretary David Davis said Britain was ready to "roll up its sleeves" to get a deal.
Mr Davis insisted the British position papers were sufficiently detailed, stating: "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: "For the United Kingdom the week ahead is about driving forward the technical discussions across all the issues.
"We want to lock in the points where we agree, unpick the areas where we disagree, and make further progress on the whole range of issues."
He said this will "require flexibility and imagination from both sides", adding: "And we are ready to roll up our sleeves and get down to work again once more."
Ahead of the talks, Mr Davis was believed to be frustrated at Mr Barnier's insistence that progress must be made on fixing the UK's "divorce" bill before moving on to talks on future trade.
He believes the series of papers produced by his Department for Exiting the EU over the past fortnight have proved that the two issues are inextricably linked.
However, Brussels is understood to be infuriated at Britain's refusal even to spell out how it thinks its liabilities to the EU should be calculated, let alone put a figure on the final bill, variously estimated at between £50 billion and £80 billion.
The latest round of withdrawal talks began in the wake of Labour's move to promise to keep the UK in the single market and customs union after March 2019.
Under the new policy unveiled by shadow Brexit secretary Keir Starmer, the UK would continue to observe EU rules on migration and trade during a transition period expected to last up to four years, in order to avoid a "cliff-edge" shock to business.
The move was welcomed by trade unions and opponents of a hard Brexit, but sparked fury among Labour supporters of EU withdrawal, who warned it could cost the party votes in the Midlands and North.
Mr Davis's bargaining hand may have been weakened by Labour's move, which raises the prospect of possible setbacks for the Government in votes on the EU Withdrawal Bill when MPs return to Westminster in September.
Ahead of the Brussels talks, Mr Barnier used a French newspaper article to state Brexit would have an impact on defence and security matters.
Writing in Le Monde, he said: "It will have very practical consequences, including on defence and security.
"The British Minister of Defence will no longer be able to sit on the Council of Defence Ministers; London will leave the European Defence Agency and Europol; British defence research centres and the defence industry will not be eligible for the European Defence Fund; London will not be able to assume the command of European operations.
"Nevertheless, the union of 27 and the United Kingdom will have to join forces to deal with common threats: the safety of our fellow citizens is not being marketed.
"We will therefore examine in due course the conditions for convergence between the union of 27 and the United Kingdom on security and defence matters."