Theresa May has warned MPs they can "move forward together" or "open the door to yet more division and uncertainty" by rejecting the UK's EU-endorsed Brexit deal.
The PM has also written to the public for support on the day the 27 EU leaders rubber stamped the proposed divorce deal in Brussels.
Her letter to the nation came amid growing opposition in Parliament that aims to thwart the deal's passage.
The prime minister will face the Commons on Monday, where she will tell MPs "with absolute certainty" that "there is not a better deal available".
EU leaders meanwhile queued up to express their sadness at Britain's impending departure - though Mrs May refused to join them.
How was the EU endorsement announced?
European Council president Donald Tusk confirmed the 27 European Union leaders have endorsed the Brexit deal after meeting for less than an hour.
His tweet came barely 40 minutes after the meeting started.
He and the EU leaders spoke with a tinge of sadness as they addressed the impact of the breakup.
How did EU leaders respond with sadness?
The European Commission president Jean-Claude Juncker said "divorce is a tragic moment."
He added: "There is, between us, something which has the remainings of love."
In a show of friendship, European Council president Donald Tusk said: "Regardless of how it will all end, we will remain friends until the end of days and one day longer."
Ahead of the summit however, European Commission president Mr Juncker warned Britain cannot expect to get a better Brexit deal if Parliament rejects the agreement hammered out by Theresa May.
He told reporters: "This is the deal. It's the best deal possible and the EU will not change its fundamental position when it comes to these issues," he said.
Mr Juncker's comments were echoed Dutch prime minister Mark Rutte who urged MPs to give their approval in next month's expected "meaningful vote" in the House of Commons.
"This is the deal on the table. I don't think there is anything more now. I don't want to contemplate a no vote. I think there will be a yes vote," he said.
"I think this is the best we can all do - both Theresa May and her Government as well as the European Union.
"I do think she has everything now to argue for a yes vote in the British Parliament."
The EU's chief negotiator Michel Barnier said the deal was a "necessary step" to prepare for the next phase of negotiations which the Government hopes will result in a wide-ranging free trade agreement.
"Now it is time for everybody to take their responsibility. This deal is a necessary step to build the trust between the UK and the EU we need to build," Mr Barnier said.
"The next phase is an unprecedented and ambitious partnership. We will remain allies, partners and friends."
What was the PM's response to their breakup messages?
Mrs May didn't see the consent of the UK's Brexit deal as a moment of sadness, as ITV News Europe Editor James Mates noted.
The prime minister insisted she was "full of optimism about the future" after achieving the "best deal available" in 18 months of negotiations.
She then fielded a series of questions about the likely domestic opposition which could rule the deal defunct.
But she refused to accept an alternative course will need to be taken.
How has Theresa May appealed for support?
Ahead of the meeting Mrs May penned a "heart and soul" letter urging the nation to back her Brexit deal.
The almost 800-word message is an attempt to speak directly to the public to build support for her deal, which faces widespread opposition.
Westminster including from both wings of her Conservative Party and the Democratic Unionist Party which props up her administration.
She said the deal "will honour the result of the referendum" by allowing the UK to "take back control" of its money, laws and borders.
"It is a deal for a brighter future, which enables us to seize the opportunities that lie ahead," she said.
She promised she would be "campaigning with my heart and soul" to win the vote in the Commons.
And "with Brexit settled" the UK will be able to focus on the economy, NHS, building homes and tackling the "burning injustices" in society, the Prime Minister added.
The summit in Brussels is only taking place after Spain claimed the UK and EU had given into its demands for concessions over the future of Gibraltar.
Spanish Prime Minister Pedro Sanchez claimed the UK and European Union had agreed to its demands for guarantees over the status of Gibraltar in future negotiations.
What lies ahead for Mrs May?
The Commons vote is the major hurdle Mrs May must now overcome if her hard-won deal is to be enshrined in UK law.
The vote is expected to take place before MPs break for Christmas in December.
However, the DUP, Labour, SNP and the Liberal Democrats have said they will vote against it in December.
On Sunday, Jeremy Corbyn described the deal as a "miserable failure" and "bad for the country".
If the withdrawal agreement is rejected by MPs it could mean a general election or the UK leaving with no deal unless Mrs May attempts to renegotiates another deal.