In his draft guidelines, Tusk proposed that if the 27 leaders saw "sufficient progress" towards a withdrawal treaty then negotiations on a future trading relationship could start.
He added "probably in the autumn, at least I hope so". Tory euroscpetics added that they were encouraged he had shown a hint of compromise.
It will take a summit of the 27 leaders, however, to signal that that moment has come to look toward the future.
"And it will mean that as we seek a comprehensive new economic partnership with the European Union, our allies will know that we start from a position where we have the same standards and rules".
Speaking Friday at a press conference in Malta, Tusk said coming negotiations over two years on Britain's exit will be "difficult, complex and sometimes even confrontational".
The final version of the guidelines will not be approved until the summit next month, and an European Union official told City A.M. they are likely to be "tweaked and changed" before then.
That will also bring up the question of the jurisdiction of the European Court of Justice, and whether Britain will consent to be bound by future ECJ rulings on disputes over the acquired rights of EU citizens that might arise in the future. The EU's draft guidelines for Brexit are uncompromising and firm.
Diplomats say they will reflect the 27's previous insistence on a "balance of rights and obligations" - notably that Britain will have less access to European Union markets if it opts out of the bloc's system of centralised rules and common budget contributions. Populist voices in Britain have urged the Conservative government not to pay a penny.
The tiny British overseas territory on Spain's southern tip has always been the subject of an acrimonious sovereignty row between London and Madrid, which wants Gibraltar back after it was ceded to Britain in 1713.
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These include the timeframe for wrapping up the EU-U.K. talks, which remains unclear but has been an early sticking point ahead of negotiations. "There is a lot of goodwill, a lot of willingness to achieve what the Prime Minister says she wants to achieve, which is an orderly transition then a deep and special partnership between a strong European Union and strong United Kingdom".
The proposal, issued by EU Council President Donald Tusk, argues for a "phased approach" in talks and sets the tone for the two-year divorce negotiations.
Tusk also ruled out the suggestion that there was an inherent threat in British Prime Minister Theresa May's departure letter, which some felt hinted at a link between sharing security information and getting a good deal.
"I do not expect we'll bargain with security topics because it's in our common, in our mutual interest to exchange information if it's necessary", she told BBC2's Newsnight.
Tusk said agreement also had to be reached on the rights of expatriates, the border between Ireland and the provinces of Northern Ireland, and business rules before any talks on a post-Brexit trade deal could take place.
Also under scrutiny will be what Tusk says on the terms of a transition period, on the assumption a free trade pact can not be concluded for some years after 2019.
"We really are moving forward now".
In her letter, the prime minister expressed her belief that "it is necessary to agree the terms of our future partnership alongside those of our withdrawal from the European Union". "Today my task is to propose draft negotiation guidelines to 27 leaders - to the 27, because from Wednesday after triggering Article 50, the United Kingdom is now on other side oft the negotiating table".