London, July 20 (IANS): A hacking group has threatened to release intimate details of millions of Britons who use an online dating service to "cheat" on their partners, a media report said on Monday.

The Impact Team -- a shadowy group of computer hackers -- has claimed to have accessed the personal details of nearly 37 million users of the hookup service (www.AshleyMadison.com) aimed at people who are already in a relationship, Mirror Online reported.

The group has threatened to release nude images and even details of secret sexual fantasies of the members of Ashley Madison website if it is not closed down by its owners Avid Life Media.

"Avid Life Media has been instructed to take Ashley Madison and Established Men offline permanently in all forms, or we will release all customer records, including profiles with all the customers' secret sexual fantasies and matching credit card transactions, real names and addresses, and employee documents and emails," a statement from the hacking group read.

"Users almost always pay with credit card; their purchase details are not removed as promised, and include real name and address, which is, of course, the most important information the users want removed," hackers said.

Confirming this, Ashley Madison chief executive Noel Biderman said his firm was "working diligently" to get the revealing information taken down off the internet.

"We apologise for this unprovoked and criminal intrusion into our customers' information," a website spokesperson said on Monday.

An investigation into the incident has been launched.