Britain will lose more than the European Union from its decision to leave the bloc, the EU's top diplomat said on Thursday, and talks with London were expected to be difficult.

Prime Minister Theresa May formally began Britain's divorce from the EU last month, declaring there was no turning back, ushering in a tortuous exit process that will test the bloc's cohesion and pitch her country into the unknown.

Britain now has two years to negotiate the terms of the divorce before it comes into effect in late March 2019.

The EU's foreign policy chief, Federica Mogherini, said during a visit to Beijing the negotiations would be difficult.

“They will have to dismantle their belonging to a community. We will lose an important member state,” she told students at the elite Tsinghua University.

“Let me tell you that to me all member states are important, equally, because one can be contributing more on some policies than others. But I think our British friends will lose more than what we lose,” she said.

Brexit negotiations will start as planned in June, after Britons vote in a snap general election, the EU said on Wednesday. Britain's parliament has endorsed May's call for a vote on June 8.

“It's clear in our treaties that it is two years, only two years, from when the negotiations start, that was March this year. This cannot be delayed. I do not expect that it will be faster than that,” Mogherini said.

China has looked on with some concern at the Brexit process, nervous about instability in the bloc that is China's largest trading partner. It will lose around a sixth of its economic output when Britain leaves the EU.

In pictures: European parliament Brexit discussions Show all 12 1 /12 In pictures: European parliament Brexit discussions In pictures: European parliament Brexit discussions European commission member in charge of Brexit negotiations with Britain, French Michel Barnier listens at the President of the European Commission Jean-Claude Juncker speaking at the European Parliament in Strasbourg Getty Images In pictures: European parliament Brexit discussions Frank-Walter Steinmeier, President of the Federal Republic of Germany, delivers his speech at the European Parliament in Strasbourg EPA In pictures: European parliament Brexit discussions European Union's chief Brexit negotiator Guy Verhofstadt, President of the Group of the Alliance of Liberals and Democrats for Europe (ALDE), addresses the European Parliament during a debate on Brexit priorities and the upcomming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions Michel Barnier, European Chief Negotiator for Brexit reacts during a meeting at the European Parliament in Strasbourg EPA In pictures: European parliament Brexit discussions Member of the European Parliament and former leader of the anti-EU UK Independence Party (UKIP) Nigel Farage wears socks with Union Jack flag at the European Parliament in Strasbourg Getty Images In pictures: European parliament Brexit discussions Nigel Farage, United Kingdom Independence Party (UKIP) member and MEP, addresses the European Parliament during a debate on Brexit priorities and the upcoming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions European commission member in charge of Brexit negotiations with Britain, French Michel Barnier gestures during speeches at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions The President of the European Commission Jean-Claude Juncker (L) speaks with European commission member in charge of Brexit negotiations with Britain, French Michel Barnier at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions European Union's chief Brexit negotiator Guy Verhofstadt, President of the Group of the Alliance of Liberals and Democrats for Europe (ALDE), addresses the European Parliament during a debate on Brexit priorities and the upcomming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions Member of the European Parliament and former leader of the anti-EU UK Independence Party (UKIP) Nigel Farage gestures during speeches at the European Parliament in Strasbourg, eastern France, on April 5, 2017. The European Parliament will on April 5 lay down its "red lines" for negotiations over a Brexit deal, on which the assembly will have the final say in two years' time. / AFP PHOTO / Sebastien BozonSEBASTIEN BOZON/AFP/Getty Images Getty In pictures: European parliament Brexit discussions German president Frank-Walter Steinmeier delivers a speech during a plenary session at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions The European Parliament in Strasbourg, eastern France Getty Images

“The European Union, even after the UK will be out, will continue to be the first market in the world, the second largest economy in the world,” Mogherini said.

“I am seeing all our partners in these months telling us that the European Union is needed, and this is the message I also get from here in China that the European Union is an indispensable partner in the world today,” she said.