© AFP Justin TALLISBrussels (AFP) - EU President Donald Tusk said Britain must first settle the key divorce issues of "people, money and Ireland" before any talks on a post-Brexit trade deal. In a letter to leaders of the remaining 27 European Union countries ahead of a summit on Saturday, Tusk said that "before discussing our future, we must first sort out our past." Former Polish premier Tusk said the "only possible approach" was phased talks in which Britain must make "sufficient progress" on the divorce issues before talks on future relations. Britain had wanted to discuss its divorce settlement with the bloc and a trade deal at the same time. The EU says the key issues are: the fate of three million EU citizens living in Britain and one million Britons resident in the EU; Britain's exit bill estimated at around €60 billion; and the fate of the border between Ireland and the British province of Northern Ireland. "This is not only a matter of tactics, but — given the limited time frame we have to conclude the talks — it is the only possible approach," Tusk wrote to the leaders. "I would like us to unite around this key principle during the upcoming summit, so that it is clear that progress on people, money and Ireland must come first," he wrote.