Images of the Duchess of Cambridge and her children are reportedly among some 3,000 images stolen in a hack of Pippa Middleton's iCloud account, according to Britain's Press Association news agency.
Both Britain's Sun and Daily Mail newspapers reported that an anonymous seller had been in contact offering the images for sale.
The Sun said the cyber thief -- who got in touch via an encrypted messaging service and used the pseudonym "Crafty Cockney" -- claimed to have images that also included Middleton at a wedding dress fitting and nude photos of her fiance, financier James Matthews.
The seller demanded "a minimum of £50,000" (around $65,000) within 48 hours, as of Friday afternoon UK time, both newspapers said.
"This isnt (sic) an auction it will be a simple process of the highest bid," the seller reportedly said.
Kensington Palace has given no comment on the reported cyber theft.
London's Metropolitan Police Service confirmed Saturday that it had received a report concerning the alleged hacking of a personal iCloud account a day earlier.
Specialist officers are now investigating, the media office said. No arrests have been made and inquiries are ongoing.
Middleton, who is the younger sister of Prince William's wife Kate, announced her engagement to Matthews in July. They plan to marry next year.
She was a bridesmaid at the royal couple's wedding at Westminster Abbey in April 2011 -- and garnered her own headlines with an eye-catching gown.
William and Catherine have sought to protect the privacy of their children, Prince George and Princess Charlotte, in an attempt to give them as normal a childhood as possible and only alimited number of photos has been released.
It's not the first time that hackers have targeted Apple's popular iCloud online data backup service in search of images. In 2014, a hacker leaked a series of pictures showing Hollywood star Jennifer Lawrence and other female celebrities in the nude.