Great Britain has officially triggered the process of leaving the European Union by handing over a letter to European Council President Donald Tusk.
Britain's ambassador to the EU, Tim Barrow, delivered the letter signed by Prime Minister Theresa May at the European Council's headquarters in Brussels on March 29.
"After nine months the U.K. has delivered Brexit," Tusk wrote on Twitter. He later posted a photo of him receiving the letter from Barrow.
"There is no reason to pretend this is a happy day," Tusk said later in a speech, insisting that the priority now was to minimize costs for EU citizens and businesses.
"We already miss you," he also said. "Thank you and goodbye."
Addressing lawmakers in London, May hailed the triggering of Article 50 as a "historic moment from which there can be no turning back."
"We are going to take control of the things that matter most to us," she added.
"Now is the time for us to come together and be united across this house and across this country to ensure that we work for the best possible deal for the United Kingdom and the best possible future for us all," the prime minister also said.
The move comes nine months after voters in Britain approved a referendum on leaving the EU, which the United Kingdom joined in 1973.
In the six-page letter, May said the referendum decision was "no rejection" of European values and that Britain wanted the EU to "succeed and prosper."
"We are leaving the European Union, but we are not leaving Europe -- and we want to remain committed partners and allies to our friends across the continent," she wrote.
Britain is the only country to invoke Article 50, which gives both sides two years to reach agreement.
The negotiations could be extended, but this would be subject to the approval of the other 27 member states.
Ahead of signing the letter, May spoke on the phone to Tusk, European Commission President Jean-Claude Juncker, and German Chancellor Angela Merkel.
The leaders "agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process," Downing Street said.
EU leaders have said they have no desire to punish Britain for leaving the bloc, but with growing anti-EU sentiment in other member states, they also do not want to make exiting seem an attractive option.
Hours before Britain officially starts the process of leaving the EU, Britain's finance minister, Chancellor of the Exchequer Philip Hammond, insisted that the government "will get a deal."
But he told the BBC that London will need to compromise during the process, saying "everybody in the EU and the U.K. is going to go into this negotiation looking to protect their own interests."
"We understand that we can't cherry-pick, we can't have our cake and eat it," he added.
EU Commissioner for Budget and Human Resources Guenther Oettinger said he expected "many difficult negotiations in the next weeks and months."
"We need each other," German Foreign Minister Sigmar Gabriel said. "We should do everything to nurture good and friendly relations with London in the future."
Within 48 hours of receiving May's letter, Tusk is expected to send draft negotiation guidelines to the other 27 member states. EU ambassadors will then gather to discuss the draft.
A summit of EU leaders on April 29 is to agree to give the European Commission a mandate to negotiate with London.