Arriving for a special summit in Brussels, European Commission President Jean-Claude Juncker, head of the bloc's executive arm, said it was a "sad day".
European leaders on Sunday signed off on their split from Britain, approving a deal that would set the United Kingdom on a new, if uncertain, road, independent from the European Union after more than four decades of membership in the political and economic powerhouse.
A summit planned for Sunday was in jeopardy when Spanish Prime Minister Pedro Sanchez at the last minute threatened to boycott the gathering of the 27 European Union members because he wanted guarantees how the British territory off Spain's southern coast would be treated after Great Britain's planned departure from the EU.
The British Prime Minister Theresa May will still have to get the deal approved by the United Kingdom parliament, where she faces a formidable task to win a majority in the 650-member House of Commons.
The nearly 800-word message is an attempt to speak directly to the public to build support for her deal, which faces widespread opposition at Westminster including from both wings of her Conservative Party and the Democratic Unionist Party which props up her administration. "Europe and the United Kingdom have accepted our demands". He said it would greatly reduce Britain's influence and ability to make independent trade deals.
European leaders have finally approved a deal on the UK's departure from the EU during an emergency summit in Brussels.
Lithuanian President Dalia Grybauskaite also said after the session: "Brexit deal endorsed, but exit process far from over".
"I am totally convinced this is the only deal possible".
The Spanish government, Sánchez said, aims to have "fruitful, close and ambitious" relationship with the territory in the future, that will favour both Gibraltar and the so-called "Field of Gibraltar".
Liberal Democrat Brexit spokesman Tom Brake claimed the PM had "caved in" and "appears to have cast the people of Gibraltar aside".
"This is the deal on the table", she said.
Spain, which had threatened to boycott the Brussels summit, chose to attend the summit in Brussels after a disagreement over Gibraltar was resolved on Saturday.
Her foreign minister, Jeremy Hunt, said that the Brexit deal was a "staging post" towards Britain getting everything it wanted from leaving the European Union, but that the arithmetic for getting the deal approved was looking "challenging". I'm inviting those who have to ratify this deal in the House of Commons to take this into consideration.
British Prime Minister Theresa May said in Brussels, "We have worked through the withdrawal issues for Gibraltar in a constructive and sensible way".
Forged during 17 months of tough negotiations, the withdrawal agreement covers financial matters, citizens' rights, Northern Ireland and arrangements for a 21-month post-Brexit transition phase. He told reporters that the summit "is neither a time of jubilation nor of celebration".
Chief Minister Fabian Picardo fired back at Spanish Prime Minister Pedro Sanchez on Saturday night, dismissing his claims that Spain had secured "historic" gains in its sovereignty aspirations over Gibraltar.
Sanchez said the agreement reached would give Spain "absolute guarantees to resolve the conflict that has lasted for more than 300 years before Spain and the U.K".
He says her proposed Brexit agreement reached with the European Union would leave the U.K.in a "pitiful and pathetic place".
"This isn't about me", she said.
Then Britain issued a statement saying it would continue bilateral talks with Spain after Brexit on March 29 - and Sanchez relented.
Tajani said a "large majority" of European parliamentarians supported the deal.
The British government wrote to Mr Tusk to say that it would not interpret its withdrawal treaty as meaning that a future EU-UK trade treaty would automatically apply to Gibraltar - but it would seek the best deal for the territory.
DUP leader Arlene Foster said a Brexit deal that includes the backstop plan is "not in the national interest".