Brexit negotiations will not proceed to haggling over a future UK-EU trading relationship by October as previously planned, the Slovenian prime minister has warned.
Miro Cerar dashed British hopes of beginning trade talks as soon as possible, suggesting that withdrawal issues in the first stage of discussions - a financial settlement, citizens' rights, and the Irish border - were too complex to solve in time.
Under the agreed timetable for negotiations, "sufficient progress" must be made on the withdrawal issues before talks on a future trade deal can begin.
Both sides hope that European Commission chief negotiator Michel Barnier will be in a position to make that recommendation to October's European Council summit of EU leaders, who will have to approve a move to second stage trade talks.
Brexit Secretary David Davis, who originally predicted the "row of the summer" over the sequenced approach to talks, but backed down, used a Sunday newspaper article to attempt to push the negotiations towards trade.
But Mr Cerar, who will be one of the 27 EU leaders at October's summit, 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.
"What is important now is that the three basic issues are solved in reasonable time.
"Then there will be optimism on realistic grounds. I know this issue of finance is a tricky one. But it must also be solved, along with the rights of people."
The Slovenian PM also criticised a British position paper published last week, which focused on potential future customs arrangements, saying it amounted to "cherry-picking", which Brussels has repeatedly warned against.
"I think it is not realistic, but in the process of negotiations every side has the right to put his proposals and the other can respond," Mr Cerar said.
"As we said at the beginning, there can be no cherry-picking. This is a very complex whole that we have to solve."
Mr Cerar's comments came as Mr Davis said some issues in the first phase of talks were "inextricably linked" to the future relationship to be discussed in phase two.
Signalling the UK's wish to move the talks forward, the Brexit Secretary wrote in the Sunday Times: "Nowhere is that point truer than on the question of Northern Ireland.
"It is simply not possible to reach a near-final agreement on the border issue until we've begun to talk about how our broader future customs arrangement will work."
This week, the Government will publish five position papers further setting out Britain's negotiating strategy in an attempt to add pace to the negotiations.
A key document is expected on the Government's favoured approaches to enforcing rights outside the jurisdiction of the European Court of Justice (ECJ).
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.
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.
It will come after weeks of speculation that the Government could favour a mechanism modelled on the EFTA court, which adjudicates on issues relating to countries outside the EU that participate in the single market, such as Norway.
Addressing the issue, Mr Davis wrote: "While we believe this will likely require a new and unique solution, our paper will examine a number of precedents.
"There is a common theme with all these examples - none of them involves the direct jurisdiction of the Court of Justice outside the EU.
"So, we're not being dogmatic in our approach but building on existing precedents to find a solution."
Sir Paul Jenkins, who was the Government's most senior legal official for eight years until 2014, has warned that Britain would have to replicate EU rules and submit to the ECJ "in all but name" if it wants to ensure there are no hard borders after Brexit.
Liberal Democrat home affairs spokesman Ed Davey said: "Either Theresa May is trying to con Brexiteers and really understands the critical legal point Sir Paul Jenkins is making - or she is heading for the hardest of Brexits, despite the evidence that this could only work at huge cost to British people."