Ministers for the European Union from the bloc's 27 remaining member states on Monday approved the terms under which chief negotiator Barnier will discuss Britain's departure.

The news was tweeted by Barnier's deputy, Sabine Weyand.

Barnier himself said he did not want to think about the talks collapsing and that both sides could move to discussing a trade deal if key issues related to the withdrawal were agreed quickly.

"We will need to make sufficient progress on this first phase if we are to move to phase two as quickly as we can at the end of this year and the beginning of next as we negotiate the future relationship between the EU and the UK," he said.

It had been expected that Barnier's mandate would be adopted without contention, with the directives based on guidelines that were agreed upon after only a few minutes of debate at an April 29 summit in Brussels.

Read more: Young Brits keep calm and try to make best of Brexit

Those guidelines laid out the red lines for any final agreement, including:

EU citizens in Britain and UK citizens in the EU should be guaranteed reciprocal treatment.

Any transitional deal should not last any longer than three years, and be overseen by the EU's Court of Justice (ECJ).

To have close trade with Europe, the UK must adhere to the EU's environment and anti-tax evasion rules.

The final deal cannot include a trade-off between trade and security cooperation.

The UK should be liable for costs that "arise directly from its withdrawal" and fulfil financial obligations already made, even after its exit.

The relocation away from London of the European Banking Authority and European Medicines Agency.

EU leaders have agreed that before talks on a trade deal, there needs to be "sufficient progress" on the rights of EU citizens in the UK and British citizens on the continent, as well as London's exit bill and border arrangements in Ireland. The EU 27 stipulate there should be no hard border between Northern Ireland and the Republic of Ireland, amid fears of the impact that Britain's exit could have on the peace process.

Britain has said it wants the talks on trade to take place parallel to the wider terms of Brexit.

Watch video 00:36 EU leaders back Brexit strategy

Three-stage plan

Barnier hopes for an agreement on the first phase by the end of this year, with the launch of a second phase between December 2017 and spring 2018, the finalization of a Brexit deal is envisaged for about October 2018.

Earlier this month, Barnier said the biggest stumbling block to a smooth Brexit could be the failure to agree on the sum Britain will have to pay when it leaves the EU, and that a methodology to calculate the amount should be agreed swiftly.

British Prime Minister Theresa May has said "the next five years will be among the most challenging in our lifetime" as the UK seeks to unravel itself from more than 40 years of EU membership. May is currently in the middle of an election campaign, seeking to enhance the parliamentary majority of her Conservative Party ahead of the talks.

Read more: When Theresa met Jean-Claude: What really happened at that Brexit meeting?

On Sunday, Britain's "Brexit Minister" David Davis said the UK was prepared to walk away from the talks without a deal, but stressed he thought it was likely an agreement would be reached.

The UK voted on June 23 last year to leave the EU, with 52 percent of those who voted opting for Britain to "Leave" and 48 percent choosing "Remain."

rc/rt (AFP, dpa, Reuters)