French President Emmanuel Macron said on Wednesday the EU could only agree to extend the Brexit negotiations if Britain gave a clear reason for requesting a delay, striking a firmer tone than Germany's Chancellor Angela Merkel.

Britain is due to exit the EU on March 29 but its parliament has still to approve the terms of the divorce. On Tuesday Prime Minister Theresa May signalled that Brexit could be delayed till June if parliament rejects an amended deal next month.

"If the British need more time, we would support an extension request if it was justified by new choices from the British," Macron told a joint news briefing with Merkel.

"But we would in no way accept an extension without a clear view on the objective pursued," he added. "As our negotiator Michel Barnier said, we don't need more time, we need decisions."

Merkel said she was "totally on the same line" as Macron but appeared more willing to show flexibility.

"If Britain needs some more time, we won't refuse but we are striving for an orderly solution," she said.

Extending the withdrawal period would need the approval of all 27 remaining EU members.

French officials have said Paris would only agree to delay Brexit if that came with a credible solution, for example if Britain called an election, held a second referendum, or presented a new plan that was acceptable to all sides but needed more time to be finalised.