Britain's negotiations to leave the European Union (EU) will be very demanding, both for the British and the remaining 27 members of the bloc, German Chancellor Angela Merkel said on Thursday.

"The exit negotiations will definitely demand a great deal of the European Union as well as Britain itself in the two years. I think that is completely beyond doubt," Merkel told Germany's Bundestag lower house of parliament.

She said she shared the view of European Commission (EC) President Jean-Claude Juncker that the Brexit negotiations could only get going properly after Britain's June 8 election.

On the other hand, Italian Prime Minister Paolo Gentiloni said on Thursday the 27 EU countries should remain united during negotiations with the Great Britain on its exit from the bloc.

Speaking in parliament ahead of an extraordinary EU summit on Saturday called to set the terms for the negotiations, Gentiloni said he expected negotiations to begin in mid-June, and he "trusted" they would lead to a positive result, in which the United Kingdom would remain a "friend and ally".

"It's crucial that the 27 countries remain united," Gentiloni said. "If the 27 countries are divided and each negotiates independently in its own interest, then it will kill all possibility of an accord."