Brussels, Belgium: The European Union urged British Prime Minister Theresa May on Friday to start Brexit talks as quickly as possible but warned of complications ahead after she lost her majority in a snap election.

As May promised to stick to the timetable for starting negotiations in 10 days' time, the EU said time was running out to secure a deal before Britain formally leaves in March 2019.

European Council President Donald Tusk said the "urgent task" was to conduct talks "in the best possible spirit" and minimise disruption for citizens and businesses.

"The timeframe set by Article 50 of the Treaty leaves us with no time to lose," Tusk, who acts on behalf of the EU's 28 national leaders, said in a congratulatory letter to May, referring to the divorce clause in the EU's Lisbon Treaty.

Former Polish premier Tusk had earlier called on May to "do your best to avoid a 'no deal' as result of 'no negotiations'", after Britain's repeated threats to walk out of talks if the EU pushes key issues including a 100-billion-euro exit bill.

Brussels was left in shock after May's gamble to increase her majority badly backfired, with fears that it could hold up talks that have still not started almost a year after Britain voted to leave the EU.

Despite facing calls to resign, May vowed Friday to form a new government to lead Britain out of the EU, adding after a speech in Downing Street: "Now let's get to work."

European Commission chief Jean-Claude Juncker said he hoped there would not be "further delay" in the Brexit talks that "we are desperately waiting for."

"As far as the commission is concerned we can open negotiations tomorrow morning at half past nine," Juncker told reporters in Prague.

EU Brexit negotiator Michel Barnier, who was responsible for setting the goal of starting talks in the week beginning June 19, said they should start when the "UK is ready."

"Timetable and EU positions are clear. Let's put our minds together on striking a deal," the Frenchman said on Twitter.

Barnier had previously set a timetable of talks starting the week beginning June 19, with agreement on initial issues by autumn of this year and a provisional Brexit deal in October 2018.

But EU budget commissioner Guenther Oettinger said Britain's May was now likely to be a "weak" partner.

"The British need to negotiate their exit but with a weak negotiating partner, there is a danger that the talks are bad for both parties," Oettinger told German radio.

- 'Clock is ticking' -

Britain and the EU are at odds over almost every detail of the divorce, from the sequencing of the talks, to the rights of EU citizens living in Britain and the size of the bill Brussels says it must pay to leave.

EU officials had hoped a strong win for May would make it easier to override domestic opposition and compromise with the EU. 

European Parliament Brexit negotiator and Belgian ex-premier Guy Verhofstadt warned the election result would make talks more difficult.

"Yet another own goal, after Cameron now May will make already complex negotiations even more complicated," Verhofstadt tweeted, comparing the outcome to former British prime minister David Cameron's ill-fated decision to call last year's Brexit referendum.

Manfred Weber, the head of the European Parliament's largest group and a key ally of German Chancellor Angela Merkel, said May had caused "chaos".

"EU is united, UK is deeply split. PM May wanted stability but brought chaos to her country instead," tweeted Weber, who leads the centre-right European People's Party.

"The clock is ticking for Brexit. Therefore the UK needs a government soon. The date for the beginning of negotiations is now unclear."

Gianni Pittella, leader of the socialist bloc in parliament, added: "It's a disaster for May. Her huge gamble has backfired spectacularly. She has no credibility in UK or Europe. She should resign."

May must now face the other 27 European leaders at a summit in Brussels on June 22 and 23.