British Prime Minister Theresa May has started the formal process that will lead to the U.K. leaving the European Union — known as Brexit — prompted by last June’s referendum.

On Wednesday, May sent a letter to Donald Tusk, President of the European Council, instructing him to trigger Article 50 of the Lisbon Treaty. This starts negotiations over the terms of the departure that will end in two years’ time, when Britain will leave the E.U., whether there is an agreement or not.

Later on Wednesday, May will address Members of Parliament, urging the country to unite. She is expected to tell lawmakers: “It is my fierce determination to get the right deal for every single person in this country. For, as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can — and must — bring us together.”

Among the many issues to be resolved in the E.U. talks will be the fate of E.U. nationals living in the U.K. and Britons living in the E.U. states; and the terms of trade between the U.K. and E.U. states, including the question of tariffs and other barriers to trade.