Estate agent Foxtons is allegedly investigating a possible data breach after claims it was hacked and its users' confidential details posted online.
Appearing on Pastebin and several other sites, the list of MyFoxtons user details included email addresses, user names and passwords.
Wired reports that Foxtons sent an email to its MyFoxtons users saying: "We have been able to download the list of usernames and passwords that were posted and are currently running checks to determine its veracity."
Computerworld UK contacted Foxtons, but was told that the estate agent would not be issuing a statement at this time.
MyFoxtons accounts contain no financial details, but home address' and telephone numbers could be retrieved if accessed.
Commenting on the alleged breach, Ross Parsell, Director of Cyber Security at Thales UK, said companies and the industry need to collaborate on cyber security issues.
"The recent spate of high-profile data breaches, such as this alleged attack on Foxtons, are evidence that organisations are either not taking cyber security seriously or are bewildered by the problem," he said.
"Regulation in this case is a necessity to alter corporate behaviour. Once the full extent of the cyber threat is uncovered, greater collaboration on cyber issues should lead to an improvement in cyber awareness and cyber standards."
He added: "In order to make this work there needs to be a holistic approach that tightly integrates cyber-defences with processes, people and physical measures. A cyber security model that can provide a useful indication of the resilience of defences and areas of vulnerability as an important guide through the prime consideration of balancing between price, safety and security, is crucial."