It comes as a senior EU official said the UK was living in a "let's just keep everything we have now, fantasy".
A UK official described those remarks as ‘laughable’ and warned against ‘trying to insult us’.
Meanwhile the UK said the EU should repay £1 billion if it is excluded from the Galileo satellite navigation system.
In a briefing, following three days of Brexit talks, a senior EU official told journalists the UK was in a fantasy that everything could stay as it is, which would mean that the EU would have to change so that Britain could remain the same.
"I'm a bit concerned because the precondition for fruitful discussions has to be the UK accepts the consequences of its own choices," the official said.
"The sooner we move beyond 'let's just keep everything we have now', the sooner we move away from this fantasy, then the quicker we can make progress."
On the issue of the Irish border, the official said "we are running out of time" and that there had been no agreement, in three days of talks this week, on the ‘crunch items’ of customs and regulatory alignment between Northern Ireland and the Irish Republic.
The UK government has proposed a fallback option if the UK and EU cannot agree new customs arrangements in time, to avoid a ‘hard border’ in Ireland. It would see the UK remaining aligned with the EU customs union for ‘a limited time’ after 2020 — something Theresa May has said would only be needed in "a very limited set of circumstances".
But the EU official said any solution must be ‘Northern Ireland-specific’ and they must "do away with fantasy that it can be a whole-of-UK solution" because that would provide a back door for the UK into the EU's single market.
The official also suggested the row over the UK's access to the Galileo system was about the UK wanting to turn it from an EU program into a joint UK/EU program which was ‘a big ask’ and would give the UK more influence than some EU member states.