The EU on Thursday dismissed British "fantasy" on solving the Irish border problem after Brexit talks in Brussels this week and warned it would be hard to make progress by a key summit in June.
The warning by a senior EU official came as UK lawmakers said Britain may have to extend its post-Brexit transition period as far as 2023 to avoid a hard border on the island of Ireland.
The official, briefing reporters on condition of anonymity, said that they were "concerned" by the fact that Britain did not seem to accept the consequences of its decision to leave the EU.
"I have the impression the UK thinks everything has to change on the EU side so everything can stay the same for the UK," the official said, summing up three days of talks with British negotiators.
Brussels has said there needs to be progress on a solution by the June 28-29 EU summit to prevent the return of border checks between EU member Ireland and the British province of Northern Ireland.
"Progress on Ireland seems elusive, substantive progress even more so," the official said.
The EU has proposed a "backstop" solution under which Northern Ireland would remain under the bloc's rules, but Britain says that would effectively create a border between the province and the rest of the UK.
British counter proposals for the whole of the country to stay in the EU's customs union after it leaves the bloc -- while leaving other EU regulations and the control of the bloc's top court -- are in return rejected by Brussels.
"We have to do away with the fantasy that there is an all-UK solution" of Britain staying in the customs union to avoid a hard Ireland border, the official said.
Britain also had unrealistic expectations on the extent of a future defence relationship, and on its wish to stay in the Galileo satellite navigation programme, the EU official added.
The official meanwhile said that the British negotiators had not formally mentioned any extension of the current transition period, which is due to run from Britain's departure in March 2019 until the end of 2020, and refused to comment on press reports.
Britain's Brexit parliamentary committee said in a report Thursday that the government's failure to draw up a Brexit blueprint may mean the country will be forced to remain in the EU's customs union past 2020.
British Prime Minister Theresa May's cabinet is in deadlock over two options for customs arrangements -- although the EU official said they expected formal British proposals within weeks.