Photo: The Canadian Press

The United Kingdom filed for divorce from the European Union on Wednesday, overturning four decades of integration with its neighbours, 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.

To Britain, he said: "We already miss you."

For Britons who voted 52 to 48 per cent 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. "Things have been bad round here for a while and we needed a change."

Former U.K. Independence Party leader Nigel Farage, who campaigned for years to take Brexit from fringe cause to reality, said Britain had passed "the point of no return."

"I can still, to be honest with you, scarcely believe today has come," he said.

Others worried Britain was taking a leap in the dark.

"No one knows. No one knows how this is going to go," said Nicola Gibson, who works in London's financial district. "It's a gamble, it's a risk. It's a bit like Trump. It's such an uncertain place at the moment."