The EU's approach to the talks which will establish Britain's new relationship with the remaining 27 member states is to be set out by European Council president Donald Tusk.
Following the triggering of the Article 50 withdrawal process by Theresa May on Wednesday, Mr Tusk is to circulate his draft guidelines for the forthcoming negotiations to the other EU leaders.
Even before the talks have got under way, a row has erupted between London and Brussels as to how they should be conducted.
On Thursday French President Francois Hollande backed Germany's Chancellor Angela Merkel in insisting the terms of the separation - including the so-called "divorce bill" - must be settled before they can move on to discuss a new trade relationship.
That puts them directly at odds with Mrs May who said in her formal Article 50 letter to Mr Tusk that she wanted the two negotiations to proceed in parallel.
Although the issue sounds technical, it could have a crucial bearing on the outcome as Britain is likely to have less leverage in the trade talks if it has already agreed how much it is going to have to pay to leave the EU.
Mr Tusk, who is attending a gathering of EU centre-right leaders in Malta, issued a thinly veiled warning to the UK on Thursday not to try to play divide-and-rule with the other 27.
"Brexit has made us, the community of 27, more determined and more united than before. I am fully confident of this," he said.
His guidelines - which are expected to be formally adopted at an EU summit at the end of April - are expected to endorse the position of France and Germany that trade talks must wait until the separation has been agreed.
EU negotiators have indicated that they will be looking for a payment of around £50 billion to cover Britain's outstanding obligations.
While Mrs May has acknowledged there will have to be a "fair settlement" UK ministers have been adamant it will not be anything like the sort of sum being talked about in Brussels.
Meanwhile there was continued irritation over Mrs May's apparent attempts to link continued security co-operation with a new trade deal.
The Prime Minister has warned that if the UK is forced to leave the EU without an agreement future co-operation on tackling terrorism and crime would be "weakened", prompting accusations of "blackmail".
Downing Street denied that it was attempting to use Britain's intelligence-gathering capabilities - regarded as the strongest in the EU - as a lever in the negotiations.
However German defence minister Ursula von der Leyen insisted it was in both sides "mutual interest" to continue intelligence-sharing and that it would not be a major element in the negotiations.
"I do not expect we'll bargain with security topics because it's in our common, in our mutual interest to exchange information if it's necessary," she told BBC2's Newsnight.
"There's a good and long tradition of working together in security topics. So I think this will not be the hotspot in negotiations. It's going to be about trade and the common market, and those are the things where the beef is."