360-million accounts that are available for sale on cyber black markets,that it uncovered stolen credentials by a cybersecurity company.

The discovery could represent more of a risk to consumers and firms than stolen credit card data because of the chance the sets of user names and passwords could open the door to online bank accounts, corporate networks, health records and virtually any other type of computer system.

Hold Security chief information security officer Alex Holden said that his firm obtained the data over the past three weeks, meaning an unprecedented amount of stolen credentials is available for sale underground.

"The sheer volume is overwhelming," said Mr Holden, whose firm last year helped uncover a major data breach at Adobe Systems in which tens of millions of records were stolen.

He said he believes the records were obtained in separate attacks, including one that yielded some 105-million records, which would make it the largest single credential breaches known to date.

Mr Holden said the firms attacked may remain unaware until they are notified by third parties who find evidence of the hacking. "We have staff working around the clock to identify the victims," he said.

Mr Holden has not provided any information about the attacks to other cybersecurity firms or authorities but intends to alert the companies involved if his staff can identify them.

The massive trove of credentials includes user names, which are typically e-mail addresses, and passwords that in most cases are in unencrypted text.

He said that in contrast, the Adobe breach, which he uncovered in October last year, yielded tens of millions of records that had encrypted passwords, which made it more difficult for hackers to use them.

The e-mail addresses are from major providers such as AOL, Google, Microsoft and Yahoo and almost all Fortune 500 companies and nonprofit organisations.

Mr Holden said he alerted one major e-mail provider that is a client, but he declined to identify the company, citing a nondisclosure agreement.