Britain's Brexit Secretary has told the top EU negotiator that London could accept legally-binding guarantees on the disputed Irish border backstop, without requiring changes to the already-negotiated EU-UK divorce deal, diplomatic sources said.

That would run against the official line of British Prime Minister Theresa May, who has said she needs changes to the legally binding Withdrawal Agreement to secure ratification of the accord by the British parliament.

But diplomatic sources on both sides said after a meeting between the two men earlier this week, Brexit Secretary Stephen Barclay still sought a unilateral exit clause from the backstop for Britain or a firm time-limit. 

The EU has repeatedly ruled out both. 

Barclay is due in Brussels on Monday for another meeting with chief EU negotiator Michel Barnier.

Meanwhile, Irish Prime Minister Leo Varadkar said it is possible Britain will ask for an extension of its March 29 deadline to exit the European Union.

"I don't think it's inevitable; it's certainly possible. If there is going to be an extension, it needs to be with a purpose, it needs to be with a view to securing and ratifying an agreement,” Varadkar told reporters on Friday.

"I don't think anyone would like to see this stalemate or impasse or period of purgatory continue for months and months and months."

Varadkar has also reiterated the backing of the European Union for Ireland on Brexit.

"One of the most striking things about what's unfolded since the UK's decision to leave has been the remarkable solidarity from the EU side, despite many attempts to bilateralise issues," he said at a Brexit conference in Dublin on Friday.

"The solidarity has been strong and resolute and those who think it will break at the last moment are in for a nasty surprise."