Legendary singer Elton John announced "with the heaviest heart", he would have to cancel a show in the US city of Indianapolis due to being "extremely unwell".

John made the announcement on Saturday and the concert was suppose to take place on the same night.

The heaviness presumably extends well beyond the disappointment of not being able to play for fans: Gladys Furnish, mother of his husband David Furnish, died Thursday, reports variety.com.

John acknowledged the death at his show in Toronto that night.

Attendees say he told the crowd that the woman he called the matriarch of the Furnish family had died at 3 a.m., sending condolences to her other sons and her husband before dedicating "Don't let the sun go down on me" to her.

In the message he posted to fans on social media on Saturday afternoon, John did not mention her passing, but made it clear that he was in no shape to go on with the show.

"To my dear fans in Indianapolis: It is with the heaviest heart that I'm forced to deliver the news that I am extremely unwell and therefore unable to perform at Bankers Life Fieldhouse tonight," he wrote.

"I absolutely hate to let my fans down, but I owe it to y ou to put on the best show possible and unfortunately that's simply no t possible. The date will be rearranged for March 2020, and I promise I will deliver the show you deserve. Thank you so much for your support and understanding, Elton x."

There's no word on whether the next stop on his farewell tour, Monday at Nashville's Bridgestone Arena, will be affected.