It is one of the red line issues which the UK must address before Brussles will start discussing trade terms
EU leaders are meeting the UK halfway and committing to holding trade talks before the final details of its exit deal have been signed off on.
However, it will not immediately open parallel exit and trade deals - it will only talk trade when "sufficient progress" has been made in relation to divorce negotiations.
British officials have argued that trade talks and exit talks should be held at the same time to ensure that deals can be agreed to within Article 50's two-year time limit.
Mr Tusk says negotiations must happen between all of the EU and the UK - not individual nations
"Once and only we have achieved sufficient progress on the withdrawal can we discuss the framework for our future relationship," European Council President Donald Tusk said.
"Starting parallel talks on all issues at the same time, as suggested by some in the UK, will not happen," he cautioned.
This will not happen until certain 'red line' issued have been dealt with, including:
Mr Tusk said it is "only fair" that the UK should honour financial promises it made as part of the 28-member bloc.
He added that there would be, "no punitive approach, Brexit is punitive enough."
The Council President stated that the UK cannot pursue bilateral talks with EU nations, saying it should negotiate with "only with the (EU) 27 as a unity."
Additional reporting IRN