EU leaders will task European negotiators to prepare trade talks with Britain as soon as possible, fearing a no-deal Brexit, according to a draft document seen by AFP on Thursday.

File photo: VCG

Prime Minister Boris Johnson's Conservatives are leading opinion polls for the December 12 UK election, promising a swift withdrawal from the EU followed by a finalised trade deal by the end of 2020.

Johnson has also promised not to extend the transition phase, which Britain is allowed to request for up to two years leaving more time to strike a deal.

The squeezed timetable promised by Johnson raises the threat of a no-deal Brexit, a worst-case scenario that would leave Britain with no formal links to Europe and hammer the UK's economy.

The UK vote takes place on the first day of a two-day EU summit in Brussels, and the remaining 27 leaders are set to discuss the election aftermath over lunch the following day.

A senior EU diplomat called the trade deal timetable "nearly impossible" and a draft of the communique to be released by the leaders ordered officials to get to work at once.

Negotiators, led by the EU's Michel Barnier, must draw up a mandate "for a future relationship with the UK immediately after its withdrawal", said a draft of the summit conclusions seen by AFP.

"Negotiations should be organised in a way that makes the best possible use of the limited time available for negotiation and ratification by the end of the transition," said the text, which was still open for changes by diplomats.

EU capitals are extremely nervous about Johnson's schedule, especially Germany, which backs a softer line towards Britain.

"If we fail to conclude this agreement in the transition period, which will be most challenging... we face the continued threat of a de facto hard Brexit," German Ambassador Michael Clauss said at a conference in Brussels on Monday.

But a diplomat told AFP that some countries will push back against hurrying, insisting that "we have time".

"We don't want speed to become our guiding principle because we don't want to be made responsible for a no-deal," the diplomat said.

After several delays, Britain's new departure date is January 31, though the main opposition Labour party -- trailing in the polls -- is pledging to renegotiate the divorce deal and put it to a second referendum.

If the divorce deal is passed, the UK has until July 1 to ask the Europeans for a delay to the year-end transition.