Brexit deal 'best possible' and 'not renegotiable': Macron

French President Emmanuel Macron said Tuesday the Brexit deal is the "best agreement possible and is not renegotiable", as Britain's premier pushed to reopen talks with Brussels.

Macron's comments during a summit in Cyprus came as Prime Minister Theresa May appealed to British lawmakers to give her a mandate to renegotiate, after parliament rejected an accord reached with the European Union.

Macron urged the British government to "promptly" lay out to EU chief negotiator Michel Barnier "the next steps that will prevent an exit without an agreement, which nobody wants but for which we must all prepare ourselves".

With the deadline for Britain's divorce from the bloc just two months away, Brussels has insisted it will not budge on the December deal, which the House of Commons overwhelming rejected on January 15.

May's massive defeat heightened fears that Britain could leave the EU on March 29 with no new arrangements in place, prompting economic chaos on both sides of the English Channel.

The British premier now backs a proposal to remove a controversial "backstop" clause on Northern Ireland from the agreement, with MPs set to vote from 1900 GMT on the way forward.

She has promised that if this passes, she will seek a "significant and legally binding change to the withdrawal agreement" with the EU before Britain leaves the bloc as planned on March 29.

Amendments before the British parliament on Tuesday evening include ruling out a "no deal" Brexit, delaying exit day and returning to the negotiating table.

Brexit hardliners from May's Conservative party oppose the backstop, which could see Britain indefinitely tied to EU trade rules in order to keep open the border with the Republic of Ireland.

Following May's defeat this month, Macron said a transition period would be necessary "because the British cannot afford to no longer have planes taking off or landing at home, and their supermarkets, as much as 70 per cent, are supplied by continental Europe".

A presidential source said France was speeding up preparations for a "no deal" Brexit.