British PM May fires starting gun on Brexit

Her priorities also include leaving the jurisdiction of the European Court of Justice and securing "frictionless" trade with the bloc while ending full membership of the customs union thatsets external tariffs for goods imported into the bloc.

She wants to negotiate Britain's divorce and the future trading relationship with the EU within the two-year period, though EU officials say that will be hard.

"It was you, the British, who decided to leave, not us who wanted you to go," said one senior EU diplomat. "The trading relationship is going to be the most difficult bit to solve - Idon't see how that will be done in that time frame."

A huge number of questions remain, including whether exporters will keep tariff-free access to the single market and whether British-based banks will still be able to serve continental clients, not to mention immigration and the future rights of EU citizens in the UK and Britons living in Europe.

One major uncertainty for May is who will be leading France and Germany, which both face elections this year.

"It’s bad news for everybody. It’s a wedge pushed into the European project," French centrist presidential candidate Emmanuel Macron told Europe 1 radio.

The pro-EU Macron has struck a firm line on Brexit, saying he would ensure Britain gains no undue advantages outside the Union.