Britain’s envoy Tim Barrow will deliver it to EU President Donald Tusk on Wednesday.

British Prime Minister Theresa May on Tuesday signed a historic letter that will launch Brexit when it is delivered to Brussels on Wednesday, a photo released by her office showed.

Sitting in front of a Union Jack national flag and a portrait of Britain’s first Prime Minister, Robert Walpole, a serene-looking Ms. May signed the letter to begin the country‘s departure from the European Union.

The letter will be taken from 10 Downing Street to Brussels, where it will be delivered by Tim Barrow, Britain’s E.U. ambassador, to EU President Donald Tusk on Wednesday.

Formally notifying Mr. Tusk of Britain’s intention to leave the bloc, by triggering Article 50 of the EU’s Lisbon Treaty, sets the clock ticking on a two-year negotiating period to end the U.K.’s 44 years of membership.

Ms. May on Tuesday evening made separate phone calls to Mr. Tusk, European Commission President Jean-Claude Juncker, and German Chancellor Angela Merkel.

“They agreed that a strong EU was in everyone’s interests and that the U.K. would remain a close and committed ally,” Downing Street said. “They also agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process.”

As Britain begins its break with the bloc on Wednesday, Ms. May will address Parliament and promise to represent everyone in the U.K. — including EU citizens — in the negotiations with Brussels.

She will express the hope “that we are no longer defined by the vote we cast, but by our determination to make a success of the result”.

“We are one great union of people and nations with a proud history and a bright future. And, now that the decision has been made to leave the EU, it is time to come together,” the Prime Minister will say, according to her speech published in advance by Downing Street.

Britain voted by 52% to leave the EU in a referendum on June 23, 2016.