On the second day of a summit in Brussels, leaders will agree that "sufficient progress" has been made after a deal on citizens rights, the Irish border and Britain's outstanding payments, giving negotiators a mandate to move onto the main phase of talks.
He congratulated UK Prime Minister Theresa May on reaching what the BBC's political editor Laura Kuenssberg said was a major moment in the Brexit process.
The EU also wants more clarity from the United Kingdom on what sort of trade and future relationship it envisages after Brexit to be able to start discussions on that future after March, when EU leaders adopt another set of guidelines for those talks.
After Britain officially exits the EU on March 29, 2019 it will lose its seat on the European Council, the body representing the 28 member states.
Mrs May said the two sides would begin discussions on future relations straight away and hoped for "rapid progress" on a transitional phase to "give certainty" to business. The document leaves no doubt that a formal free trade agreement can not be signed until after the United Kingdom has left.
Labour's worldwide trade spokesman, Barry Gardiner, welcomed the move forward, but said it would be a "real problem" for business if the European Union didn't start talking trade for a further three months.
While the European Union is willing to engage in "preliminary and preparatory discussions" on trade as part of building a "close partnership" after the UK's departure, this means any formal agreement "can only be finalised and concluded once the United Kingdom has become a third country".
And time is tight.
The three-page document says the United Kingdom will remain under the jurisdiction of the European Court of Justice and be required to permit freedom of movement during any transition period.
In a separate tweet she promised "securing the greatest possible access to European markets, boosting free trade with countries across the world, and delivering control over our borders, law and money" in the Brexit deal.
The document "calls on the United Kingdom to provide further clarity on its position on the framework for the future relationship".
Speaking in London after confirmation that talks will now start in the new year on Britain's future relationship with the EU, Labor's Jeremy Corbyn said: "This should have happened months ago".
"I have no doubt that the real test of our unity will be the second phase of the Brexit talks", Tusk told reporters as he arrived at the summit.
Paul Masterton, a potential Tory rebel who abstained in the defeat on Wednesday, said it was a "sensible amendment to the Withdrawal Bill that gives us flexibility on Brexit date if we need".
European Commission President Jean-Claude Juncker said Friday that "some of us thought, including me, that she did make big efforts and this has to be recognized".
But British officials are hoping for a deal by March on a two-year transition out of the bloc, during which their relationship would largely stay the same.
Praising Mrs May as a "tough, smart and polite" negotiator, he said he was "entirely convinced" that the final agreement reached would be approved by the United Kingdom and European Parliaments.