EU president Donald Tusk said the bloc already misses Britain but would act as one to defend its interests after London triggered the two-year exit process on Wednesday.
"What can I add to this? We already miss you. Thank you and goodbye," Tusk said in Brussels after receiving a letter from British Prime Minister Theresa May requesting the EU divorce.
"There is no reason to pretend this is a happy day," he said in a short statement to reporters, adding: "In essence this is about damage control.
In a historic moment at around 1:26 pm (1126 GMT), Tusk accepted the letter from Tim Barrow, the British ambassador to the EU, as they shook hands against a backdrop of EU and Union Jack flags.
But the former Polish premier added that the European Union would stay together during the talks on Britain's exit.
"Paradoxically there is also something positive in Brexit. Brexit has made us, the communuity of 27 more determined and more united than before," he added.
"Today I can say that we will remain determined and united, also in the future, during the difficult negotiations ahead," he added.
The remaining EU 27 members issued a statement through Tusk saying they would priotise an "orderly" divorce -- in contrast to May saying in her letter that she wanted to discuss new ties and the split in parallel.
"In these negotiations the Union will act as one and preserve its interests," they said in the statement.
"Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and Member States," they added.
"Therefore, we will start by focusing on all key arrangements for an orderly withdrawal," they said.
The EU27 said they would "approach these talks constructively and strive to find an agreement" and added that "we hope to have the United Kingdom as a close partner" in future.
Tusk will issue more detailed political guidelines for the Brexit negotiations, which EU leaders will sign off on at a summit on April 29.