Canadian film-maker Sarah Polley has shared a "cruel" April Fools' joke played on her by none other than her 11-year-old child.

A letter turned up on Saturday morning, reading "We say this to you with the deepest regrets: the Oscar you received was given by mistake."

Polley won the best-adapted screenplay for Women Talking at the 95th Academy Awards in Los Angeles last month.

The letter, posted on Twitter, asked her to "mail it back" to California.

It said she could keep the award for one more week so she could "enjoy its presence" in her home.

But ultimately, it needed to be returned so it could go to the "rightful" winner: All Quiet on the Western Front.

"We are sorry for your loss, but it is only fair that the play with the real best-adapted screenplay gets the Oscar."

Polley's child went on to joke that Oscar bosses had realised their error on the day the award was given - but wanted to avoid another blunder like La La Land being named winner of Best Picture in 2017 instead of Moonlight.

The filmmaker behind All Quiet on the Western Front, Edward Berger, got in on the joke.

"To save on mailing costs as I live overseas the Academy has asked me to provide you with my address so you can ship the Oscar directly," he tweeted to Polley.

"I will follow up shortly. Ok with you?"

Polley, who shot to fame as an actor in the 1990s, swiftly realised the letter was not written by the Academy, but by her child as an April Fools' Day prank.

But she made it clear she wasn't impressed - saying her 11-year-old "swung low" for April Fools' Day.

"We feel it is wrong you get this on 1 April as you will probably think it is a joke, and we feel that is wrong, so another letter will be sent assuring you that this is not a joke," the letter said.

"This is much too cruel to be a joke, ergo we deeply apologise for any inconvenience we may have caused you."