The European Union is not trying to keep Britain in and wants to start discussing future ties the moment the UK parliament approves Brexit, partly to focus on its own unity ahead of May elections, the head of the bloc's executive said.

"It is being insinuated that our aim is to keep the United Kingdom in the EU by all possible means. That is not our intention. All we want is clarity about our future relations.

"And we respect the result of the referendum," Jean-Claude Juncker (photo above), the head of the European Commission, told German newspaper Welt am Sonntag in an interview.

Juncker said the EU was ready to start negotiating a new deal with Britain right after the British parliament approves the divorce deal. A vote is now due in the week starting Jan 14.

He also said Britain should get its act together.

"And then tell us what it is you want," he said.

"I am working on the assumption that it will leave, because that is what the people of the United Kingdom have decided," he added, refusing to be drawn into whether Britain would hold a second Brexit vote. "That is for the British to decide."

On other challenges facing Europe, Juncker said he was watching closely US President Donald Trump on trade.

"I trust him for as long as he keeps his word. And if he no longer keeps it, then I will no longer feel bound by my word either," Juncker said of tensions between the EU and Washington around car tariffs.

He said he felt EU citizens were increasingly growing apart, another problem to tackle ahead of Europe-wide parliamentary elections in May.

"We have to ensure that these rifts do not become too deep," Juncker said. "We must not imply that the populists are right... they are just loud and do not have any specific proposals to offer on solving the challenges of our time."

He said Europe had to stand united "in combatting the trolls and hacker groups from China or Russia" that could seek to sway the European vote.

He expressed doubt about EU state Romania, which takes over the bloc's rotating presidency from Jan 1, but struggles with corruption and bitter divisions.

"The government in Bucharest has not yet fully understood what it means to take chair over the EU member states ...

"Romania's internal situation is such that the country cannot act as a compact unit in Europe," Juncker said.

- Reuters