EU ministers voiced caution Monday about the chances of striking a rapid Brexit deal, at the start of another week billed as crucial for an agreement.

Ministers from the other 27 member states were to be briefed by EU chief negotiator Michel Barnier as time runs down before Britain's withdrawal on March 29.

"We have time but not so much," Belgian Foreign Minister Didier Reynders told reporters as he arrived for talks with fellow EU ministers in Brussels.

"It's difficult to make real progress, but before Christmas I'm hoping it's possible," Reynders said, as the EU awaits a "positive signal" from London.

If Barnier is not able this week to report "decisive progress" the chances of a November European summit to sign off on deal will begin to evaporate.

Martin Callanan, Britain's junior minister in the Department for Exiting EU, also voiced caution.

"We can't rush it," Callanan told reporters. "This is an agreement that will endure for many years. We have to make sure we get it right."

But Callanan admitted the deal must be agreed in time for the British and EU parliaments to approve it before March 29.

British Prime Minister Theresa May will host a weekly cabinet meeting Tuesday, amid reports that she has a draft withdrawal agreement to show her ministers.

The agreement will finalise the country's exit bill of around £39 billion ($50 million), guarantee citizens' rights and see a 21-month transition phase -- during which London will follow EU rules.

During the transition, British and European negotiators will launch more ambitious talks to agree the future trading and legal relationship.

The withdrawal agreement must include a "backstop" to prevent the return of a hard land border between Northern Ireland, part of the UK, and the Republic of Ireland if the sides fail to find a free trade pact.

Barnier was to meet one-on-one with Irish Foreign Minister Simon Coveney, then head to Strasbourg, for meetings with EU parliamentarians.

"Negotiations are at a very critical and sensitive stage," Coveney said before leaving for Brussels.

"We want an agreement to be reached as soon as possible but urgency is required," he said, stressing the need for a "legally operable backstop."

May has drawn the fury of her Northern Irish allies the Democratic Unionist Party -- which opposes the backstop -- and of eurosceptics in her own party.

The DUP props up her government and fears being left in a customs union with the EU.

Former British foreign secretary Boris Johnson slammed May's plans, saying she was "on the verge of total surrender" and urging her ministers to mutiny.

"We are already looking at the biggest failure of UK statecraft since Suez," the Brexit-supporter wrote for his employer, The Daily Telegraph.

"Even if the cabinet mutinies -- as they ought -- it will make little difference."

Jo Johnson, Johnson's Remain-backing brother, resigned as transport minister on Friday, branding May's proposed withdrawal deal a "terrible mistake".