A series of high-profile stars including Jennifer Lawrence, Rihanna and Jenny McCarthy have fallen victim to one of the biggest celebrity privacy breaches in history, resulting in photographs and videos apparently showing them in the nude being widely circulated on the internet.

The photographs of mainly female celebrities were shared online after an anonymous hacker using the name Tristan posted what was claimed were authentic images on the 4chan website, an online message board used for sharing pictures.

Some of the celebrities targeted immediately said the images were fake, while others confirmed their veracity and said they had been taken in private and had never been shared.

The 4chan website quickly removed the hacker’s posts, but a screenshot of the list of hacked celebrities included pop star Avril Lavigne and actors Kirsten Dunst, Aubrey Plaza and Winona Ryder, while experts have warned that more images could emerge over time. Other alleged victims include the models Candice Swanepoel, Cara Delevingne and Kelly Brook, and the TV host Cat Deeley. A number of photographs, including images of Lawrence, star of The Hunger Games, have been circulating on file-sharing and image-sharing sites. The hacker also apparently posted images of his computer desktop, which included what appeared to be an image of Lawrence. Lawyers for the actor immediately threatened to prosecute any publication publishing the pictures. A spokesman for Lawrence said: “This is a flagrant violation of privacy. The authorities have been contacted and will prosecute anyone who posts the stolen photos of Jennifer Lawrence.” The actor Mary Elizabeth Winstead confirmed that the photos of her were real and said they had been taken in private with her husband and had been deleted before they were posted by the hacker. She condemned the actions of voyeurs who were continuing to access the images since news of the hack broke.

“To those of you looking at photos I took with my husband years ago in the privacy of our home, hope you feel great about yourselves,” she wrote on Twitter. She added: “Knowing those photos were deleted long ago, I can only imagine the creepy effort that went into this. Feeling for everyone who got hacked.”

In a statement, the lawyer for model and actor Kate Upton said: “This is obviously an outrageous violation of our client Kate Upton’s privacy. We intend to pursue anyone disseminating or duplicating these illegally obtained images to the fullest extent possible.”

The American actor and singer Victoria Justice, who was also named in the file, denied any nude photos of her were real in a tweet on Monday, writing: “These so called nudes of me are FAKE people. Let me nip this in the bud right now. *pun intended*.”

The gymnast McKayla Maroney wrote that “the fake photos of me are crazy!!”, while a spokesman for Ariana Grande said photographs of her were “completely fake”.

It remains unclear how the images and videos, which were not publicly available, were accessed by the hacker. One expert, who had seen the files, said the oldest photos date back to December 2011 and the newest from 14 August.

It was initially suggested that Apple’s iCloud service could have been compromised to access the images, but experts have said this was unlikely. Independent security expert Graham Cluley told the Guardian suggested that a hacker could have worked for years to gather information leading to the images, or could have hacked an address book with celebrity emails and then used phishing techniques, where users are tricked into divulging their password by fake emails. “My suspicion is that this isn’t an iCloud security flaw as such,” he said.

Some security analysts have suggested that an account belonging to Kate Upton’s boyfriend could have been hacked, that a Dropbox account could have been hacked, or that a hacker who had been accumulating images for years had their system broken into by another hacker.

Others warned that more images could be released, with the owner of the Twitter account InfoSec Taylor Swift remarking: “Mark my words: Releases of these photos are going to be a slow-burn shitshow for days – probably longer.”

The celebrity gossip columnist Perez Hilton initially posted censored and uncensored versions of some of the photographs on his site, but has since removed them. “I acted in haste just to get the post up and didn’t really think things through. I’m sorry,” he tweeted. Another tweet stated: “Upon further reflection and just sitting with my actions, I don’t feel comfortable even keeping the censored photos up. I am removing them.”

The Los Angeles police department told the Huffington Post that it had no knowledge of the hacking, while the FBI said it could neither confirm nor deny reports of the attack. Apple has made no comment regarding a possible security breach.