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".
It took Britain's Theresa May and 27 other European Union leaders just 40 minutes to sign the Brexit deal after two years of tortuous negotiations, but the trials and tribulations of Britain's withdrawal agreement approved Sunday in Brussels are far from over.
British Prime Minister Theresa May will then have the momentous task to sell the deal to her recalcitrant Parliament and a nation still fundamentally split whether the United Kingdom should leave the European Union on March 29 and under what conditions.
But given that opposition parties - not to mention many lawmakers within May's Conservative party and the Northern Irish DUP, which supports her minority government - have indicated they'll vote against it, the deal's biggest test is yet to come.
The comments were echoed by Johnson, who received a rapturous applause from the delegates, saying Britain would be left as a "satellite state" by the deal.
In a letter, UK's ambassador to the EU, Sir Tim Barrow, said the Withdrawal Agreement imposes no obligations regarding the "territorial scope" of future agreements.
"I am totally convinced this is the only deal possible", he said.
On a visit to Cuba, Sanchez said he had yet to receive assurances that any future decisions on Gibraltar would be decided in direct talks between London and Madrid.
The DUP's 10 MPs have proved reluctant to vote with the Government since the terms of the Brexit deal became known and the termination of their Westminster arrangement would be a major blow to the Prime Minister.
British Prime Minister Theresa May, waiting in the wings as the other leaders met, joined them once they had given the deal the seal of approval to discuss the next steps on the road to Brexit.
The deal must still be ratified by the European Parliament, something that its president, Antonio Tajani, said would likely take place early in 2019.
Theresa May insisted the UK's position on Gibraltar "has not changed" after she was accused of caving in to Spanish demands ahead of Sunday's Brexit summit.
Eurosceptic Tory MP Andrew Bridgen, a prominent critic of the Prime Minister, said: "With regard to the Withdrawal Agreement and the proposed future relationship "never in the history of our great nation have so many been betrayed, so deliberately, for so little".
May declined to say when asked by a caller whether she would resign if the deal was rejected by Parliament.
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.
British Ambassador to the EU, Tim Barrow wrote in a letter that the United Kingdom. European leaders resolved a last-minute dispute over the future of Gibraltar on November 24, clearing the way for a summit to approve the Brexit deal.
Twenty-seven European Union leaders have endorsed a deal that sets out the terms of Britain's departure on March 29 and sets a framework for future ties.
Britain could also simply crash out on March 29.
"This isn't about me", she said.
Tajani said a "large majority" of European parliamentarians supported the deal.
In an open letter to the British public published Sunday, May promised to campaign "with my heart and soul to win that vote and to deliver this Brexit deal".
Senior Conservative backbencher Iain Duncan Smith was one of those criticising the Prime Minister's deal Sunday morning, telling media outlets in a series of interviews that after so much had been given away to the European Union it would be hard for him and others to vote to ratify the deal in Parliament.