Tens of thousands of Irish adulterers who signed up to cheating website Ashley Madison could be exposed.
More than 40,000 Irish men and women are members of the website whose tagline is "Life is short, have an affair".
Ashley Madison encourages users to cheat on their partners and has been targeted by a group of hackers calling themselves The Impact Team.
The hackers claim to have access to Ashley Madison's database of more than 37 million members, as well as financial records and details of user's sexual fantasies.
They are threatening to publish this information online unless the site closes down.
"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," the hackers said in a statement.
Two other websites, Cougar Life and Established Men, have been com- promised in recent months.
The hacker's main point of contention appears to be that Ashley Madison charges a leavers' fee of €21 to carry out a "full delete" of users' profiles.
While it is possible for users to remain anonymous while using the site, Ashley Madison says the delete service is the only way to completely, and permanently, remove data from its servers.
The Impact Team say this is a "complete lie", adding that details such as real names and addresses are never deleted.
The hackers claim the "full delete" has netted parent company Avid Life Media €2.7m.
ALM believes it has identified the perpetrator of the hack, which it says was probably an inside job.
The company has apologised for this intrusion which it is describing as an act of "cyber terrorism".
"We apologise for this unprovoked and criminal intrusion into our customers' information," it said in a statement.
"The current business world has proven to be one in which no company's online assets are safe from cyber-vandalism."
More than 70pc of Ashley Madison users are male.
The site has been criticised in the past for "fat shaming" plus-sized models in its billboard ads.