The UK and European Union (EU) face a “furious race against time” to finalise Brexit talks before March 2019, the head of the European Council says.

Donald Tusk urged EU leaders to show unity as they try to negotiate what the future relationship will look like and to set up transitional arrangements, BBC reported on Tuesday.

The EU is set to agree this week that enough progress has been made so far to move on from the first phase of talks.

The UK has been told not to “backtrack” on last week’s divorce deal.

The comment from EU chief negotiator Michel Barnier came after UK Brexit Secretary David Davis suggested the divorce agreement unveiled by Theresa May amounted to a “statement of intent” rather than a binding agreement.

Davis — the UK’s Brexit secretary — said he was quoted out of context.

But European Parliament negotiator Guy Verhofstadt said the “unacceptable remarks” would harm “good faith” in the process.

The UK is set to leave the EU in March 2019, two years after May served formal notice of Brexit.

Both sides hope to finalise a deal by October 2018 on the future relationship, including trade, so the UK and European Parliaments have time to vote on it before the UK leaves.

In his formal letter on Tuesday inviting leaders to this week’s EU summit, Mr Tusk told the 27 member states: “This will be a furious race against time, where again our unity will be key.”

On Sunday, Davis said guarantees on the Northern Ireland border – included in a joint EU-UK report published on Friday — were not legally binding unless the two sides reached a final deal.

But he told LBC Radio on Monday they would be honoured whatever happened.

A European Commission spokesman said the first-phase deal on the Northern Ireland border, the divorce bill and citizens’ rights did not strictly have the force of law.

“But we see the joint report of Michel Barnier and David Davis as a deal between gentlemen and it is the clear understanding that it is fully backed and endorsed by the UK government.”