A 35-year-old man has been arrested by British police investigating claims that Pippa Middleton's iCloud account was hacked and 3000 intimate photos were stolen.

Scotland Yard said they arrested the man on Saturday evening at an address in Northamptonshire.

The theft of 3000 photographs including images of Middleton's sister, the Duchess of Cambridge, and her children, Prince George and Princess Charlotte.

Naked photos of her millionaire fiance, James Matthews, were among the thousands of photos stolen, according to The Sun.

The photos were offered on WhatsApp to reporters at both the Daily Mail and The Sun by an anonymous seller.

The images included some of the royal children as well as at a wedding dress fitting and at parties.

The Sun said it had been approached by a person using a pseudonym and asking for £50,000 ($85,000) within 48 hours.

It says the seller communicated using the pseudonym "Crafty Cockney" on an encrypted messaging service and sent sample photos showing Middleton being fitted for a wedding dress in advance of her planned 2017 nuptials.

In July it was announced that the 33-year-old would marry her financier boyfriend, James Matthews, 40, next year.

The Sun says the hacker also claimed to possess Middleton's informal photos of sister Kate with her children, Princess Charlotte and Prince George, and naked images of her fiance.

According to the report in The Sun, the hacker said he was offering them to buyers in the UK first, adding: "There is of course a price for which im (sic) accepting offers of a minimum of 50,000 GBP. This isnt (sic) an auction it will be a simple process of the highest bid in the next 48 hours (as of 4pm 23/09/16)".

A spokesman for Middleton thanked media, saying: "Thank you very much for drawing this to the family's attention. I can confirm that not only have the lawyers been informed but the police are about to be involved as well."

Officers at Scotland Yard were contacted on Friday night (Saturday morning AEST) and were investigating.

Fairfax Media