The European Union's chief Brexit negotiator Michel Barnier said the bloc was prepared to offer Britain an unprecedentedly close relationship after it quits the EU, but it would not permit anything that weakened the body's single market.

"We are prepared to offer Britain a partnership such as there never has been with any other third country," Barnier told reporters in Berlin on Wednesday after a meeting with German Foreign Minister Heiko Maas, adding that that could include economic as well as foreign and security policy ties.

"We respect Britain's red lines scrupulously. In return, they must respect what we are," he said. "Single market means single market ... There is no single market a la carte."

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