In an interview run by the website of the British newspaper on Monday, Cerar said 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.

According to the Slovenian prime minister, the opening talks have shown that the two sides are too far apart on citizens' rights, financial settlement and the Irish border. The Guardian notes that Cerar is the first EU leader to publicly admit what has been increasingly feared in the UK.

"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," he said.

Cerar added that what was important now was that the three basic issues were solved in reasonable time. "I know this issue of finance is a tricky one. But it must also be solved, along with the rights of people."

Cerar also suggested that 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 Guardian explains.

Cerar said the initial schedule set out by the EU's chief negotiator Michel Barnier had always looked optimistic, but he suggested the UK had not helped its cause by seeking to "cherrypick".

The newspaper says that Cerar's comments seem to suggest that the second phase of talks will be delayed until December or January. In such case, Britain will be left with ten 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, it adds.