The new head of the European Council of national leaders Charles Michel, will host European Commission president Ursula von der Leyen and their 27 national counterparts on Thursday and Friday.

The 28th seat at the table will be empty. Prime Minister Boris Johnson faces a general election battle on Thursday, and as results come in overnight his counterparts in Brussels will be following his bid to win a working majority to deliver a Brexit divorce deal.

A clearer picture of whether Britain's new parliament will be able to pass the withdrawal agreement and take the country out of the EU on January 31 should emerge before the second day of the summit, and EU leaders want to waste no time in launching talks on a future trading relationship.

According to a draft of the summit's conclusions seen by AFP, the 27 leaders will ask Von der Leyen's commission to draw up a "draft comprehensive mandate for a future relationship with the UK immediately after its withdrawal", to guide EU negotiator Michel Barnier in trade talks.

On the campaign trail Johnson has boasted he has a deal "ready to go" and will not need to extend Britain's post Brexit transition beyond the end of next year. But Europe will have its own priorities, in particular access to British fishing waters and a "level playing field" for business.

There is scepticism in Brussels that anything more than a bare-bones arrangement can be agreed in time. France's President Emmanuel Macron on Tuesday stressed the importance of EU countries agreeing and sticking to a united position and being ready shortly after the election.

"We must be able to move forward quickly," he said, after talks with Michel.

He said it was important to stick with the method followed so far - maintaining unity in the EU position, allowing Michel Barnier alone to conduct negotiations, and defending EU interests without yielding to pressure while also respecting fair competition with Britain.

- Budget 'massacre' -

Macron is keen to see the withdrawal agreement finally ratified to end the uncertainty Britain's difficult, oft-delayed departure has caused EU firms -- and also to allow leaders to concentrate on his own plans for deeper European unification and reforms in Brussels.

But the summit, at which Von der Leyen will present plans for a European Green Deal low-carbon economy, will also lay bare stark differences between the 27 powers on the union's long-term seven-year budget and ambitions to build a carbon neutral economy by 2050.

Brussels hopes new technology will reduce the emissions that drive climate change, and carbon capture techniques -- such as forest-planting -- will absorb the rest. But as late as June three coal-dependent member states opposed the 2050 target: the Czech Republic, Hungary and Poland.

Von der Leyen is nevertheless planning legislation, which could be drawn up by the ene of February, to bind member states to the plan -- along with a "transition mechanism" that fossil-fuel users like Poland will include cash payments to smooth their transfer to renewable energy.

But in order to set aside money, the EU would have to have an agreed budget framework for the 2021-2027 period -- and Thursday' summit debate on this promises to be intense -- a "massacre" in the words of one EU official.

The draft budget suggested by the previous European Commission in June last year was too expensive for many members states. Cuts proposed last week by Finland, holder of the union's rotating presidency, have upset countries like France with new spending priorities.