EU leaders on Wednesday welcomed clarity from Britain over what it wants from Brexit talks, with Germany's Angela Merkel promising a united front in what will be "very intensive" negotiations.

Responding to Prime Minister Theresa May's speech on Tuesday that made clear that Britain would quit the European Union's single market, Merkel welcomed the fact that London had accepted that free movement of EU citizens was not something it could opt out of without losing other EU rights.

"There cannot be any cherry picking by Britain in Brexit negotiations," Merkel told an economic conference, adding that access to the bloc's single market was ultimately linked to accepting the EU's four freedoms - of movement of goods, capital, people and services.

"The speech made by British Prime Minister Theresa May has given us a clear impression of how Great Britain wants to proceed," Merkel said at a joint news conference with Italian Prime Minister Paolo Gentiloni.

"The main thing is that Europe does not let itself be divided and we will make sure of that via very intensive dialogue," Merkel said.

Gentiloni said the EU was "ready to discuss the issue with the correct approach, which is in solidarity and friendship with the UK".

A spokesman for the German finance ministry said London's vision for implementing Brexit could prove to be "pretty complicated" given that Britain would have no "EU passport" to conduct financial transactions once it left the bloc.