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, according to Reuters.
"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.
"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.