Miro Cerar says after opening talks the two sides are too far apart on citizens’ rights, financial settlement and the Irish border

The British government’s hopes of opening discussions on a future trade relationship this autumn will definitely be dashed by the European Union due to the slow progress of Brexit negotiations, one of 27 prime ministers who will make the decision has said.

Miro Cerar, the prime minister of Slovenia, revealed in an interview with the Guardian that it had proved too difficult to close the differences between the two sides in the opening rounds of talks, with the UK producing some unrealistic proposals.

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In October the European council, on which Cerar sits, will decide by unanimity whether sufficient progress has been made on the three key issues of citizens’ rights, the financial settlement and the Irish border, in order for talks to be widened to negotiations over future trade once the UK has left the bloc.

Writing in the Sunday Times, the Brexit secretary, David Davis, reiterated his calls for such broader discussions to begin, insisting the issue of the Irish border is intrinsically linked to whatever future customs arrangement is struck with the EU.

“I firmly believe the early rounds of the negotiations have already demonstrated that many questions around our withdrawal are inextricably linked to our future relationship,” Davis said. He added: “Both sides need to move swiftly on to discussing our future partnership, and we want that to happen after the European council in October.”

Cerar – the first EU leader to publicly admit what has been increasingly feared in Downing Street – said he was awaiting a briefing from the EU’s chief negotiator, Michel Barnier, but that it had become clear in recent weeks that the threshold for wider talks would not be met this autumn as had been planned.

The Slovenian prime minister said: “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 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.”

Brussels has been particularly frustrated by Britain’s refusal to offer any proposals of what it is willing to pay in the divorce deal. The foreign secretary, Boris Johnson, has not helped to ease concerns among the EU27 after he suggested that they could “go whistle” if they sought some of the figures being suggested. It has been mooted the figure could be close to €100bn (£91bn), although it is likely to be significantly lower.

Anand Menon, director of the thinktank UK in a Changing Europe, said Brussels was highly unlikely to accede to Britain’s request to discuss a future partnership alongside the withdrawal deal, because Barnier has a fixed negotiating mandate, agreed by all member states.

“The irony of this is, the Brexiters were absolutely right: the EU is very immovable and un-agile, because it has to please 27 countries. Now we’re on the receiving end of it. It is very hard for them to shift their negotiating position,” Menon said.

Should the second phase of talks be delayed until December or January, as Cerar’s comments seem to suggest, Britain will be left with 10 months at best to find agreement on a transitional deal to prevent a cliff edge for business in March 2019, when the UK leaves the EU. In that period they will also need to scope out how a future comprehensive trade deal might work, before any withdrawal agreement is put to the European parliament and the House of Commons.

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Having to put off trade talks until 2018 will be a huge and embarrassing blow for the British government, which at one stage said it was going to insist on negotiating the withdrawal agreement – including the issues of citizens’ rights, the divorce bill and the Irish border – alongside the post-Brexit trade agreement.

Davis had said he would generate “the row of the summer” by insisting on the two sets of talks taking place in parallel, only to be forced to accept the EU’s demands for sequential talks on day one of the first negotiating week.

Despite that failure, Davis claimed only last week that the talks had been going “incredibly well”, and that the UK’s apparent failure to offer a clear picture of what it wanted from the talks was merely a clever tactic, which he described as “constructive ambiguity”.

Those comments may yet rebound on Davis. Two Brexit papers published by the government last week, on customs and Northern Ireland, had been intended to force the pace but appear to have done the reverse.

In particular, Cerar suggested, the UK had been naive in its proposals with regard to a future customs arrangement. The British government has suggested it will leave the current customs union but agree on a new deal that will give the UK all the current benefits plus the ability to negotiate free trade deals with the rest of the world.

The EU leader said the initial schedule set out by Barnier had always looked optimistic, but he suggested the UK had not helped its cause by seeking to “cherrypick”, despite insistence by Brussels that such an approach would be rebuffed. “Since this is the initial phase, and all three [issues] are very demanding, they still need a little bit more time for a solution,” he said. “It [the schedule] was optimistic. It was good we made a schedule but, of course, if you are realistic you cannot expect such fundamental issues to be solved in a few months.

“I read the first commentaries [on the UK’s customs proposals]: ‘This is unrealistic, no way, etc’. 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. As we said at the beginning, there can be no cherrypicking. This is a very complex whole that we have to solve.”

The government has signalled that it will publish another five position papers this week, including on the thorny issue of which body will police Britain’s future trade deal with the EU, given the government’s rejection of European court of justice (ECJ) jurisdiction.

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The shadow Brexit secretary, Keir Starmer, said if that issue is not settled satisfactorily, it could hamper the wider negotiations. “The prime minister’s ideological obsession that there should be no role whatsoever for the ECJ or any court-like body will continue to hinder the prospect of any meaningful and lasting arrangement with the EU.

“We have already seen that this unnecessary and dogmatic position has prevented a sensible approach to issues such as Euratom and citizens’ rights. If the prime minister does not change course and show more flexibility on this central issue, it will haunt her throughout the Brexit negotiations.”

Davis’s pledge in his Sunday Times article to end the “direct jurisdiction” of the ECJ was interpreted by some lawyers as a softening of the official UK position on withdrawing from the ECJ.

Accepting an indirect link would deprive the UK of any legal influence at the Luxembourg court, but might preserve a relationship with the EU’s highest court through a dispute resolution body similar to that operated by the European Free Trade Association (Efta), it was being suggested.

Carl Gardner, a barrister, former government lawyer and legal commentator, tweeted: ‘Interesting. He’s now only against the ECJ’s “direct jurisdiction”, whatever that means. Maybe he’ll accept something like the Efta court.’

Davis himself appeared before the ECJ in 2016 as a claimant in a landmark civil liberties surveillance case. Although he personally had to withdraw when he became Brexit secretary, the court subsequently found in favour of the claim.

Davis’s opposition to the ECJ may therefore be less doctrinaire and more pro-business than the prime minister’s apparent hardline antipathy to any European judge, whether sitting in Luxembourg or Strasbourg.