European Union leaders will meet without Theresa May to discuss the future of the bloc after Brexit.
The Prime Minister left the summit in Brussels after the conclusion of the first day's agenda, clearing the way for the other 27 leaders to carry on in her absence.
It is expected to be the final gathering of the EU27 before Mrs May formally starts the Brexit process, which she has promised to do by the end of the month.
At a press conference before leaving the Belgian capital, the Prime Minister said: "Our European partners have made clear to me that they want to get on with the negotiations. So do I.
"It is time to get on with leaving the European Union and building the independent, self-governing, global Britain the British people have called for.
"And so, as I have said, we will trigger Article 50 by the end of this month. This will be a defining moment for the UK as we begin the process of forging a new role for ourselves in the world - as a strong country with control over our borders and over our laws."
But she signalled she was ready for a fight over demands for a Brexit "divorce bill" which could potentially run to tens of billions of pounds.
Mrs May said: "I'm clear that the way people voted on June 23 for us to leave the European Union, they voted for us in the future not paying huge sums of money into the European Union every year.
"And of course when we leave the EU that will be the case."
The European Commission's chief Brexit negotiator Michel Barnier refused to comment on the prospect of the UK paying billions into the EU budget as he arrived at the summit.
But he later revealed that he agreed a "common position" on the budget with Bulgarian President Rumen Radev, who will hold the rotating EU presidency for the first half of 2018 - potentially a crucial period in negotiations.
The discussions in Brussels will pave the way for a declaration on the future direction of the EU at a gathering later this month marking the 60th anniversary of the Treaty of Rome, which laid the foundations for the modern union.
The European Commission has released an outline of five possible futures for the EU after Brexit - including one scenario where Brussels dramatically scales back its activities to focus on the single market only.