Britain's Prime Minister Boris Johnson speaks to mental health professionals during a visit to Watford General Hospital in Watford, north of London, on October 7. Johnson warned he will not delay Brexit beyond October 31, saying his latest proposals are a last chance to reach a deal, a Downing Street spokesman said. Photo: AFP

A Brexit deal could yet be reached this week, the chief EU negotiator said Monday, but member states warned they will not allow Britain to open a back door to their single market.

British and EU officials are scrambling to draft a deal on Britain's exit terms before a summit of EU leaders starts on Thursday, as Prime Minister Boris Johnson seeks to fulfill his vow to take Britain out on October 31.

European diplomats said Britain has already given ground on customs rules for Northern Ireland, but must go further quickly if a deal is to be done this month.

As he arrived in Luxembourg to brief the other 27 EU states on the state of the closed-door talks, chief EU negotiator Michel Barnier struck a cautiously upbeat note.

"This work has been intense all along the weekend and yesterday because even if the agreement will be difficult - more and more difficult to be frank - it's still possible this week," Barnier told reporters. 

"Obviously any agreement must work for everyone - the whole of the United Kingdom and the whole of the European Union. Let me add also that it is high time to turn good intentions into legal text."

After weeks of gloom and growing fears Britain would crash out of the bloc with no divorce arrangements in place, the last few days have given a glimmer of hope that an agreement can be reached - though there has so far been no decisive breakthrough.

More than three years after Britain's 2016 referendum vote to leave, talks remain stuck on how to avoid customs checks on the border between British-ruled Northern Ireland and EU-member Ireland.

The EU has reservations about London's proposed customs arrangements and the role for Northern Ireland's Stormont assembly in giving consent to the plans. 

London's Brexit Minister Stephen Barclay will join Barnier in Luxembourg later in the morning in what is being seen as a positive sign for progress in the talks - which have been shrouded in secrecy since the weekend as the two sides seek to avoid potentially damaging leaks.

"It's not done yet but we're trying. The British want a deal and they have moved on customs and Stormont," a senior European diplomat told AFP.