Prime Minister Theresa May filed formal Brexit divorce papers on Wednesday, pitching the United Kingdom into the unknown and triggering years of uncertain negotiations that will test the cohesion of the European Union.

Nine months after Britons voted to leave, May notified EU Council President Donald Tusk in a letter that the U.K. is quitting the bloc it joined in 1973.

The prime minister, an initial opponent of Brexit who won the top job in the political turmoil that followed the referendum vote, now has two years to settle the terms of the divorce before it comes into effect in late March 2019.

"The United Kingdom is leaving the European Union," May told lawmakers in the British parliament "This is an historic moment from which there can be no turning back."

On the eve of Brexit, May has one of the toughest jobs of any recent British prime minister: holding Britain together in the face of renewed Scottish independence demands, while conducting arduous talks with 27 other EU states on finance, trade, security and other complex issues.

For the EU, already reeling from successive crises over debt and refugees, the loss of Britain is the biggest blow yet.