Ursula Von der Leyen, the president of the European Commission, warned Boris Johnson at a press conference on Friday.
SHE said, “We have to bridge wide divergences which remain to be solved.
“The topics are known: level playing field, fisheries, governance, the scope of our police and judicial co-operation.
“These are important points for the EU because these are principles – fair competition, rising social standards, protecting our citizens and the rule of law – at the heart of the EU.”
Johnson, having previously delegated to his chief negotiator David Frost – is set to take part in talks himself later this month.
Von der Leyen promised the EU will do “everything” to reach a “one-of-a-kind” agreement with the UK.
Her comments came after EU leaders met via video conference on Friday for the latest European Council summit.
After leaving the EU the United Kingdom is currently in a “transition period” in which the two sides have until December 31 to agree their future relationship.
During the transition period, the UK effectively remains a member of the EU.
Last week, Cabinet Office minister Michael Gove has confirmed that the UK will not enter into another extension.