European Council president, Donald Tusk, is due to issue draft guidelines on how the EU intends to negotiate the UK’s departure from the bloc.
The proposals will be sent to the governments of 27 EU members to help set the tone for two years of tough negotiations and efforts to finalise a deal.
EU leaders say the UK must first agree its exit terms before talks can move on to future ties.
London is calling for simultaneous talks.
At a news conference, Mr Tusk said: “Starting parallel talks on all issues at the same time as suggested by some in the UK will not happen.
“Only once we have achieved sufficient progress on the withdrawal can we discuss the framework for our future relationship.”
Talks would be “difficult, complex and sometimes even confrontational”, he predicted, but the EU would not “pursue a punitive approach”.
British Prime Minister, Theresa May, formally triggered the Brexit process on Wednesday, by sending the Article 50 notification to Mr Tusk.
The European Council had intended to publish the guidelines on its website straight away, but two EU member states objected.