LONDON (AP) — The United Kingdom filed for divorce from the European Union on Wednesday, overturning four decades of integration with its neighbors, demolishing the notion that EU expansion is inevitable and shaking the foundations of a bloc that is facing challenges to its identity and its place in the world.

Britain’s top envoy to the EU, Tim Barrow, hand-delivered a letter to European Council President Donald Tusk formally triggering a two-year countdown to the final split.

“Today the government acts on the democratic will of the British people,” Prime Minister Theresa May told lawmakers in the House of Commons, adding, “This is an historic moment from which there can be no turning back.”

There is “no reason to pretend this is a happy day,” Tusk told reporters, emphasizing that the priority now is to minimize costs for EU citizens and member states.

For Britons who voted 52 to 48 percent to leave the bloc in a referendum nine months ago, it was a time for celebration.

“I voted for Brexit and today is the day that vote starts to count,” said Charles Goodacre, a former taxi driver, in the northern England city of Sunderland.