In a letter to the heads of state and government for all 28 EU member states, Tusk said his draft agreement "goes really far in addressing all the concerns raised" by Cameron, but added: "The line I did not cross, however, were the principles on which the European project is founded."
"This has been a difficult process and there are still challenging negotiations ahead," Tusk added. "Nothing is agreed until everything is agreed."
The circulation of the draft agreement kicks off another two weeks of high-stakes negotiations ahead of the European Council meeting later this month. Cameron has said that he wants to secure a final deal at the meeting, paving the way to hold an In/Out vote as soon as June.