The UK's partnership with the EU has been hailed by some of the remaining 27 member states after .
Here's how leaders responded to today's historic events:
EU President
European Council President Donald Tusk spoke to a press conference upon receipt of the letter from the UK triggering Brexit.
“There is nothing to win in this process and I am talking about both sides. In essence, this is about damage control,” the Polish top-official said.
"There is no reason to pretend this is a happy day - for me, I will not pretend that I am happy today," he said.
“We already miss you. Thank you and goodbye."
Sweden
Sweden's Prime Minister Stefan Lofven said it regretted the UK's decision, describing it as a "close and valuable partner in the EU".
He said he welcomed "the constructive approach" in May's letter and hoped the relationship between the two countries would be "as positive and mutually beneficial as possible even after withdrawal".
Germany
German Chancellor Angela Merkel said she wants Britain and the EU to be "close partners" and said member nations will conduct negotiations in a "fair and constructive manner".
She said: "I hope that the British government will also approach the talks in this spirit."
The Netherlands
Mark Rutte, Prime Minister of the Netherlands, said his country had a long history with the UK, which is one of its main trading partners.
He said it was important "the best possible" relationship was established between the EU and Britain.
Malta
Maltese Prime Minister Joseph Muscat described it as "a sad day for Europe". He said: "Nobody can rejoice about what happened today. There are consequences for everyone; the bloc will work to find a way to mitigate these consequences."
Spain
Spain's President Mariano Rajoy Brey tweeted: "The UK has officially communicated its intention to leave the EU. We are prepared. Calm, tranquillity and confidence."
He added: "We will preserve the unity of the 27 state members as we move forward with European integration. Europe is our present and our future."
Portugal
Portuguese President Marcelo Rebelo de Sousa said in a statement that Britain was its "oldest ally" and "will continue to be a European country".
Ireland
The Government of Ireland said there was "no doubt" future negotiations would be "challenging".
It added: "Ireland is well prepared for the challenges ahead. We will negotiate from a position of strength as an integral part of the EU 27 team, and will work with all our partners to achieve the best possible outcome."
Belgium
Belgium's Prime Minister Charles Michel tweeted its government "will play a constructive and active role in creating a new relationship between UK & EU", adding: "We will defend the European values."
