Britain's formal exit notification made it an unhappy day for London and Brussels with EU Commission President Jean-Claude Juncker saying he was 'deeply sad'.

European Council President Donald Tusk told reporters difficult talks about Britain's exit would simply be about limiting damage for both sides, while Mr Juncker said Britons would eventually regret their decision.

Tusk, a former Polish prime minister who chairs meetings of EU leaders, waved papers as he began a short statement in Brussels.

"So here it is. Six pages. The notification from Prime Minister Theresa May triggering Article 50 and formally starting negotiations for the United Kingdom's withdrawal from the European Union," he told reporters less than an hour after receiving the letter from the British ambassador.

"There is no reason to pretend today is a happy day, neither in Brussels nor in London. After all most Europeans, including almost half the British voters, wished that we would stay together not drift apart," he continued.

Tusk, who will share a proposal of the negotiating guidelines with the other 27 member states on Friday, said there was one positive from Brexit - that it had made the remaining EU members more determined and united than before ahead of "difficult negotiations".

Tusk said the EU's goal was to minimise the costs for EU citizens, businesses and member states, whose leaders should adopt the negotiating proposals on April 29.

"There is nothing to win in this process and I'm talking about both sides. In essence this is about damage control," he said.

Concluding his brief remarks to the media in Brussels, Tusk said he had little to add about Britain: "What can I add to this? We already miss you. Thank you and goodbye."