The European Union demanded that Britain make "sufficient progress" on its divorce before any talks on a future trade deal can start as it laid out its tough Brexit negotiating plans on Friday.

EU president Donald Tusk's draft guidelines say the other 27 countries are ready for a transitional deal after Britain's exit in March 2019, but that such an arrangement would have to be under EU rules.

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British Prime Minister Theresa May had in her letter triggering the two-year exit process on Wednesday called for talks on the divorce and a future deal to run in parallel as soon as negotiations start.

The EU has proclaimed its unity on Brexit, even as it reels from Britain's intention to become the first member state to leave in the bloc's 60-year history.

Tusk's strategy will now be sent out to the leaders of the 27 remaining EU countries ahead of a special summit on April 29 when they will agree on them so that talks can start in May.

Germany and France had already set out a united and uncompromising stance against May's demands.

Tusk's guidelines say that the EU called for a "phased approach" that prioritises an orderly withdrawal that reduces the disruption caused by Britain's departure in March 2019.

"The European Council will monitor progress closely and determine when sufficient progress has been achieved to allow negotiations to proceed to the next phase" on a future relationship," the draft guidelines say.

The EU said that no trade deal can be agreed on before Brexit takes effect.

But it is also open to a transitional arrangement after Brexit as a "bridge" to a future deal some years down the line, but said that it would have to be under EU rules.

It said it is making preparations in case talks break down.

Tusk was due to give a press conference in Malta with Maltese Prime Minister Joseph Muscat, whose country holds the EU's rotating presidency, to discuss the strategy.

The EU's chief negotiator, France's Michel Barnier, is expected to get the green light to start talks with Britain on May 22, an EU official said.

May formally notified the EU of Britain's intention to leave in a letter to Tusk on Monday that diplomats described as surprisingly conciliatory in tone for the most part.

But May's warning in the letter that failure to clinch a deal on trade would affect Britain's cooperation on terrorism and security still rankled with many.

"It's not a threat," Brexit minister David Davis told BBC radio after warnings from Brussels against using security as a bargaining chip in the talks.

The EU insists it is unified ahead of the negotiations.

"Brexit is not the end of everything, but we must make it a beginning of something that will be new, stronger and better," European Commission chief Jean-Claude Juncker said in Malta yesterday