Last time it took the European Union more than two years to reach an accord on the spending plan.
Tusk is going to ask leaders whether they are ready to pay more and to urge them to focus on defense, education and migration. The Commission is the executive arm of the union and proposes new laws.
"I'm very much re-assured by what prime minister May has said and what Northern Ireland secretary Karen Bradley has said and they've said that they stand by the Good Friday agreement and that their support for it is steadfast".
"We must take it step by step starting tomorrow", he said. Cuts to agricultural funds and payments to poorer regions have already been mooted in Brussels.
Both of those proposals will go down badly with Poland, which is now at odds with Brussels over controversial reforms to its legal system, and other eastern states that refuse to take in asylum seekers.
Writing to the leaders, Tusk is reported to have said it is "unrealistic" to anticipate a budget agreement before the 2019 European elections.
In 2014 Juncker was chosen by the so-called "Spitzenkandidat"-German for "lead candidate"-system".
"You've got to remember we are leaving the European Union on March 29 (2019), we will be out of the treaties on that day, we will not have any say in the rules that are made and, therefore, people who come after that day ought not to be allowed to have the full and permanent free movement rights".
On Spitzenkandidat, Tusk will ask whether the European Union should accept that the parliament's candidate is "automatically" going to become commission chief or whether the final say must be with leaders. They view it as a power grab by the parliament at the expense of national sovereignty.
"I am not commenting on the outcome of the Chequers meeting until I know the exact conclusions.I am not a British prime minister, it would be good for Britain if I was", he told reporters before heading into the summit, which is to discuss the future of Europe after Brexit.
"There can not be, and will not be, a guarantee that one of the lead candidates will be president of the commission", a senior European Union official said, adding however that leaders wanted to avoid any "deep conflict" with the parliament.
Jean-Claude Juncker jibed that Britain would be better off if he was in charge today as Brexit tensions escalated.
After the crunch talks at the Chequers estate, both the Brexiteers and Remainers in the Cabinet claimed to be satisfied with the result - although the staunchly anti-Brexit Chancellor Phillip Hammond was said to be "shocked", The Telegraph reports.
Juncker made the comment at an European Union summit when asked about a high-level British cabinet meeting chaired by premier Theresa May aimed at thrashing out a plan for a future partnership with the EU.