Theresa May will urge leaders of the other 27 EU states on Thursday to move on as quickly as possible to talks on Britain's future trade relationship.
The EU27 are expected to rubber-stamp the European Commission's judgment that "sufficient progress" has been made on divorce issues to move Brexit negotiations on to their second phase, when they meet in Mrs May's absence on the second day of their two-day summit in Brussels on Friday.
But the EU's chief Brexit negotiator Michel Barnier has warned that this may not mean an immediate start to the trade talks which the Prime Minister is seeking.
Mr Barnier told MEPs that the European Council would initially concentrate on the terms of a transition to the post-Brexit relationship, while he would focus on turning last Friday's deal into a legally-binding Withdrawal Agreement.
A leaked draft of a text to be considered by the EU27 leaders on Friday suggests that trade talks may not start until after a subsequent summit in March, when a further set of guidelines will be produced. And Mr Barnier himself said only that "internal preparation" work was expected on future EU/UK relations at this stage.
In comments which appeared to throw doubt on UK hopes of completing a free trade agreement in time for the expected date of Brexit in March 2019, Mr Barnier said the EU27 now needed to agree between themselves on "the framework for the future relationship".
In the latest signal from Brussels that the EU would resist giving the UK preferential access to the single market, he stressed that the integrity of the market and its freedom of movement principle would be "non-negotiable" in talks.
European Council president Donald Tusk has warned that Britain and the EU face a "furious race against time" to agree a transition deal and future trade relations within the next 10 months, in order to allow time for ratification by national parliaments before the date of Brexit.
A further hurdle was passed on Wednesday when the European Parliament voted in favour of opening the second phase.
Mrs May was due to address fellow leaders over dinner on the first day of the summit on Thursday, when she is expected to say she welcomes the prospect of moving to talks on the future trade and security relationship as soon as possible.
She will again press the case for an ambitious future relationship which she believes in the interests of both the EU and UK.
But advocates of a clean break from the EU are pressing her to resist attempts to convert last week's agreement into a legal text.
Conservative former Cabinet ministers Owen Paterson and David Jones led a call by the Leave Means Leave campaign for her to keep the door open to a "no-deal" Brexit under World Trade Organisation rules.
Arguing that this would allow the UK to boost its economy by as much as £135 billion by tearing up Brussels rules and regulations, Mr Paterson said: "The bar for any deal agreed with the EU is that it must be better than a global deal on WTO rules.
"If it fails to meet this bar, then WTO rules are the best option so that we can diverge from the EU's regulation and really reap the benefits of Brexit."
And Mr Jones added: "We must not concede anything now that puts us at a disadvantage in the next stage of negotiations. That means ensuring that the Joint Report remains a statement of sincere intent, but not a legal obligation that ties our hands."
Mrs May was due to attend a memorial service at St Paul's Cathedral marking six months since the Grenfell Tower fire before flying to Brussels.
Brexit is not top of the agenda for Thursday's talks, which will be dominated by the launch of the new Permanent Structured Co-operation scheme (Pesco), which will allow EU countries to pool defence activities.
Mrs May is expected to welcome the move, which the UK believes can boost defence investment in Europe without threatening Nato's preeminent role as the guarantor of trans-Atlantic security.