Britain files for divorce from the European Union

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 a historic moment from which there can be no turning back."

There is "no reason to pretend this is a happy day," Tusk told reporters, emphasising that the priority now is to minimise costs for EU citizens and member states.

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

May's six-page letter to Tusk was polite and conciliatory, stressing that Britons want to remain "committed partners and allies to our friends across the continent."

She said the two sides should "engage with one another constructively and respectfully, in a spirit of sincere co-operation."

But there was a hint of steel in May's assertion that without a good deal, "our co-operation in the fight against crime and terrorism would be weakened." That could be seen by some in Europe as a threat to withdraw British security co-operation if the UK does not get its way.