The European Union will stick to a timetable for preparing to launch Brexit negotiations with the UK despite the British government's call for an early general election, an EU spokesperson said.

In a surprise move on Tuesday, British Prime Minister Theresa May called for a snap general election on June 8 as the country prepares for delicate Brexit talks.

"The UK elections do not change our EU27 plans," Preben Aamann, a spokesperson for European Council President Donald Tusk, said in a statement.

Tusk will chair a summit of the other 27 EU national leaders in Brussels on April 29, where he expects them to agree negotiating guidelines he has proposed.

On May 22, governments should agree the directives that EU chief negotiator Michel Barnier needs in order to launch the formal talks, Aamann said.

Barnier has said he expects full negotiations to start in early June, although it remains unclear what the British position on that will be given the electoral timetable.

May formally triggered a two-year countdown to Brexit on March 29.

She had said in her announcement "there can be no turning back" on Britain's decision to leave the EU, something she herself had campaigned against before last year's referendum.

Tusk said on Twitter that he had a "good" telephone conversation with May after she called for the election.

In a separate tweet, the former Polish premier likened the surprising twist in the Brexit saga to a movie plot by British master of suspense Alfred Hitchcock.

"It was Hitchcock who directed Brexit: First an earthquake and the tension rises."

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Tusk was referring to a comment attributed to the late filmmaker that a good film "should start with an earthquake and be followed by rising tension".

Speaking on condition of anonymity, a diplomat from an EU member state said that May "is completely right to call these elections now" as she could benefit from a favourable political context.

"The good news on the European side is that she will be less weak to make all the concessions she will have to make" in the talks, the diplomat said, adding that such concessions would be harder to make before an election.