Downing Street has insisted the British government remains "confident" of starting the second phase of Brexit talks this autumn, after an EU leader warned that the process would be delayed.
Slovenian prime minister Miro Cerar suggested the issues in the first stage, a financial settlement, citizens' rights, and the Irish border, were too complex to solve in time to allow trade talks to begin in October, as hoped for by both sides.
His warning came as the UK prepared to publish further details of its Brexit negotiating strategy including a key paper expected later this week on plans to leave the "direct jurisdiction" of the European Court of Justice.
Dr Cerar will be one of the 27 European leaders at a Brussels summit in October who will decide whether sufficient progress has been made on the "divorce deal" to move on to discussions about the post-Brexit relationship between Britain and the EU.
He told The Guardian: "I think that the process will definitely take more time than we expected at the start of the negotiations.
"There are so many difficult topics on the table, difficult issues there, that one cannot expect all those issues will be solved according to the schedule made in the first place."
A Downing Street spokeswoman said: "We are confident that we will have made sufficient progress by October to be able to advance talks to the next phase.
"That is our aim and we are confident that we are working at a pace to be able to get to that point."
Formal Brexit talks resume for their third round in Brussels next week, but the spokeswoman stressed that officials at David Davis's Department for Exiting the EU were in constant communication with EU negotiators to move the process forward.
Disagreement over the ECJ's role was a major sticking point during July's round of talks, with the UK aghast at Brussels' insistence that EU citizens' rights should be enforced by the court after Brexit.
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This week's paper is expected to set out why the UK thinks direct ECJ jurisdiction should end and put forward different potential approaches to enforce rights after Brexit.
But it will come after a warning from Sir Paul Jenkins, formerly the government's most senior legal official, that the UK will have to accept the ECJ in all but name if it wants to achieve a Brexit without hard borders.
It will also be published amid reports that the government could favour a mechanism modelled on the Efta (European Free Trade Association) court, which adjudicates on issues relating to countries outside the EU that participate in the single market, such as Norway.
Downing Street declined on Monday to discuss what a spokeswoman described as "excitable" speculation on the issue.
Addressing the issue at the weekend, Mr Davis said: "While we believe this will likely require a new and unique solution, our paper will examine a number of precedents."
DExEU is also expected to publish a paper stating the UK's wish to include services, as well as goods, in the first phase of talks, in a move apparently designed to inject pace into the process and take in some aspects of future trade.
Meanwhile, Crawford Falconer, the government's new chief trade negotiation adviser, said the trade deals Britain could strike after Brexit would help boost global security.
Last week, the government conceded that the UK would not be able to implement any new free trade agreements under a proposed customs transition deal expected to expire around two years after Brexit in March 2019.
But Mr Crawford, who will work alongside International Trade Secretary Liam Fox from this week, wrote in the Daily Telegraph: "There is a powerful political and security element to getting this right.
"History is littered with instances of the destructive political consequences of closed markets.
"This was a lesson well understood at the end of the last century's global conflicts."
He added: "Many countries still recognise that open trade policies directed at engaging with others are at the core of any strategy to improve the global prospects for political openness and stability. They are already looking to partner with us to re-energise that agenda."
This week, the government will also publish a paper on confidentiality which will make clear its intentions on ensuring official information exchanged between the UK, EU and other member states remain protected after Brexit.
A further document on civil judicial co-operation is designed to reassure the domestic legal sector.
And a paper on data will seek to ensure that it continues to be passed between the UK and EU without disruption.
