Britain's Brexit minister on Thursday condemned "lazy thinking" about the country's divorce from the European Union, urging Brussels to be ambitious in future cooperation, particularly on security issues.

As EU leaders met in Romania to debate the bloc's future without Britain, Steve Barclay used a side event to urge them to move on from the "Nigel Farage little-Englander portrayal of Brexit".

Barclay pleaded with the EU to look beyond the bitter wrangling in Westminster over the divorce deal and focus on the potential for future cooperation.

"From my generation of politicians within the UK, Brexit was an opportunity of confidence, of optimism, a desire to be more global," Barclay said.

"It certainly wasn't the Nigel Farage little-Englander portrayal of Brexit and I think too often there is lazy thinking to be frank on Brexit and a failure to look at that longer term relationship between the UK and Europe."

Britain was due to leave the EU on March 29 but the departure has been delayed twice as Prime Minister Theresa May struggles to get her withdrawal agreement approved by parliament.

Her government is in talks with the opposition Labour Party to find a compromise to clear the logjam and allow Brexit to go ahead.

The EU summit in the Transylvanian town of Sibiu is aimed at charting the future of the union after Brexit, and in the run-up there was palpable relief among officials that Britain's tortuous exit from the bloc would not dominate discussions.

May is not attending the summit but Barclay briefed EU ministers and MEPs on the sidelines on the state of the cross-party talks as well as Britain's plans for European elections later this month.