The European Union's chief negotiator in the upcoming divorce proceedings with Britain says that parallel talks on its exit from the EU and a future trade relationship "is a very risky approach" he is bent on avoiding.

Michel Barnier told EU legislators on Wednesday that "to succeed, we need on the contrary to devote the first phase of negotiations exclusively to reaching agreement on the principle of the exit."

British Prime Minister Theresa May last week sought hand-in-hand negotiations on exit and a future relationship, while the EU Council president and EU top legislators argued against it.

The Brexit talks are expected to start in late May once the negotiating guidelines of the 27 member nations have been sealed in a mandate for Barnier.

Britain insisted again, though, that it wanted to move on to discuss the future as soon as possible.

"We would like to move the conversation onto the future relationship as soon as possible," said junior Brexit Minister Robin Walker in Strasbourg, France, where the legislature was meeting.

"The best interests of both sides of this negotiation will be served by getting on to the technical discussion about the future relationship as quickly as possible in the two years that we have available," Walker said.

Both sides have a general agreement that they want to tackle the fate of the 3 million EU citizens in Britain and some 1 million Britons residing in the other EU nations first of all.

"I really welcome the fact that the parliament and the (EU) Council have set that out as a first priority from the EU perspective as well," Walker said.

The parliament's Brexit coordinator, Guy Verhofstadt, said it was perhaps best not much positive passion was ever in the cross-Channel relationship. "It never was a love affair" instead calling it "a marriage of convenience."