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.
They will 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.
British Prime Minister Theresa May formally triggered the Brexit process on Wednesday, by sending the Article 50 notification to Mr Tusk.
Mr Tusk - who is currently in Malta at a meeting of EU centre-right leaders - is expected to send the draft guidelines to the 27 EU members on Friday morning.
The European Council had intended to publish the guidelines on its website straight away, but the BBC understands that two EU member states objected.(Source: BBC)