European Council President Donald Tusk is proposing to make an offer of a 12-month "flexible" extension to the UK's Brexit date, a senior European Union official has said.

The official said the option could be presented to British Prime Minister Theresa May at the EU summit on Brexit on 10 April in Brussels.

If Mrs May accepted, Britain would have to hold elections to the European parliament in May, the official said.

"The only reasonable way out would be a long but flexible extension. I would call it a 'flextension'," the official said.

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"We could give the UK a year-long extension, automatically terminated once the Withdrawal Agreement has been accepted and ratified by the House of Commons," the official said.

"And even if this were not possible, then the UK would still have enough time to rethink its Brexit strategy. A short extension if possible, and a long one if necessary.

"It seems to be a good scenario for both sides, as it gives the UK all the necessary flexibility, while avoiding the need to meet every few weeks to further discuss Brexit extensions," he said.

Britain's exit from the EU, nearly three years since the country voted to leave the bloc, is now in doubt because the British parliament cannot decide what exit terms it wants, just a week before the current Brexit date of 12 April.

Mrs May has written to Mr Tusk with the UK's request for a further delay to Brexit.

"The United Kingdom proposes that this period should end on 30 June 2019," Mrs May said in the letter.

Officials said a request for an extension would have to be backed by sound arguments why the EU should grant it.

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Elsewhere, France opposes giving Britain an additional extension unless it produces a "clear" plan with "credible political backing," the country's European Affairs Minister said.

Amelie de Montchalin said President Emmanuel Macron's government had "read with interest" MrsMay's letter to the EU asking for an extension until 30 June.

"As the Prime Minister rightly wrote, the current impasse is not in the best interest of neither the UK nor the EU. It cannot be allowed to continue," Ms Montchalin said.

But EU leaders had taken a "clear decision" at their last summit on 21 March, she said.

"Another extension requires that the UK puts forward a plan with a clear and credible political backing," she said.

"The European Council would then have to define the necessary conditions attached to this extension.

"In the absence of such a plan, we would have to acknowledge that the UK chose to leave the EU in a disorderly manner."

German Foreign Minister Heiko Mass said Mrs May still had "many questions" to clarify.

Dutch Prime Minister Mark Rutte, who is seen as one of Mrs May's closer European allies, also said the letter "doesn't answer" important concerns.