British Prime Minister Theresa May on Wednesday triggered Article 50 of the Lisbon Treaty, thereby formally starting the process the United Kingdom will have to follow to leave the European Union, months after a viciously divided referendum colloquially called “Brexit”. The process will take two years to complete.

May had signed a letter invoking Article 50 earlier in the day. The letter has now been hand-delivered to EU Council President Donald Tusk by Tim Barrow, the British Ambassador to EU, in Brussels. The bloc’s 27 other member nations will receive copies of the letter as well. Tusk’s first reaction to receiving the letter was, “We miss the UK already.” He added, “There is no reason to pretend this is a happy day,” and said he would inform all the member nations about the processes that now need to be followed.

On Tuesday, Scotland had voted in support of holding a second referendum for the country’s independence from the United Kingdom, giving First Minister Nicola Sturgeon the go ahead to ask the British Parliament for a vote between 2018 and 2019. Scotland had largely voted to remain with the EU, and has strongly opposed the British government’s insistence on leaving it.

“This is...about giving the people of Scotland a choice on this country’s future,” Sturgeon had said. “The mandate for a referendum is beyond question, and it would be democratically indefensible...to attempt to stand in the way of it.”

The UK had voted to leave the EU in a referendum on June 24, 2016, after which David Cameron, who had campaigned for the country to remain in the bloc, resigned as prime minister. The UK government had rejected a petition calling for a second referendum. May took charge on July 11 to implement the result of the vote along with her team.