Britain's possible request for an extension to remain in the European Union beyond the March 29 deadline will not be unquestioningly accepted, an aide to French President Emmanuel Macron said Tuesday.

"An extension is not for certain or automatic," the aide said, echoing demands by several EU officials that British Prime Minister Theresa May lay out clear reasons why a delay would be necessary.

May is scrambling to avoid a chaotic no-deal departure in 10 days' time, and EU leaders had hoped she would attend their summit meeting in Brussels on Thursday with a plan to ratify the withdrawal deal before the deadline.

But EU heavyweights Germany and France have already warned they will not agree to delay Brexit simply to save the prime minister's skin.

Macron's aide said France would assess a British demand for an extension with two criteria.

"First point: is there a plan, a strategy, to justify an extension?" the aide said.

"Second point: we need to look at the impact on the European Union." He said Britain remaining longer in the EU could mean it would take part in European Parliament elections in May, would have a seat at EU meetings and take part in decisions about the bloc's future.

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