Christopher Chaney admitted hacking into Scarlett Johansson's email account. Photo:mirror.co.uk

Christopher Chaney, of Jacksonville, Florida, admitted wiretapping and unauthorised access to a computer.

Prosecutors said Chaney had accessed the emails of more than 50 celebrities.

The most notorious incident involved nude photos Johansson had taken of herself for her then-husband that were posted on the internet.

'Operation Hackerazzi'

Chaney, 35, was jailed by US District Judge S James Otero.

"It's hard to fathom the mindset of a person who would accomplish all of this," the judge said, Associated Press reported. "These types of crimes are as pernicious and serious as physical stalking."

Chaney said: "I don't know what else to say other than I'm sorry. I could be sentenced to never use a computer again and I wouldn't care."

Johansson, Aguilera and the Friends With Benefits star, Mila Kunis, agreed to waive protection of their identity in order to highlight the issues surrounding such internet hacking.

Chaney, who initially pleaded not guilty to the charges, was arrested in October 2011 as part of a year-long FBI investigation of celebrity hacking that authorities dubbed "Operation Hackerazzi".

Chaney admitted he hacked into the personal emails of his targets by clicking on the "forgot password" feature of their accounts and answering security questions by using publicly available information about celebrities that he found on the internet.

Once in control of the accounts, he obtained private photos, business contracts, scripts, and other documents.

He also went through contacts lists to find other victims and sometimes posed as friends to request more private photos.

Chaney then forwarded many of the images he found, including nude photos of Johansson, to gossip websites which posted them online.

Johansson told Vanity Fair last December that the photos were meant for her then-husband Ryan Reynolds.