An investigation has been launched into data recovered from a computer in the US found to contain information from hacked computers based in the UK.
The Met's Computer Crime Unit carried out the investigation
The Metropolitan Police is looking into the data, stolen via a computer virus, believed to have been extracted from more than 2,300 UK-based computers.
E-mail addresses, passwords and credit card and online transaction details were part of the information recovered.
Detectives are taking steps to notify urgently the victims where possible.
It is thought that a large number of victims live outside of London but all are based in the UK.
Banks and other online services, as well as internet service providers, are also being alerted.
Victims are being notified via email by the Met's Computer Crime Unit.
A detective from the Met's Computer Crime Unit said the information had been "harvested from the computers by a type of malicious code known as a Backdoor".
"It is too early to establish at this early stage how the computers have been infected," he said.
"However, there are thousands of computer users worldwide who have had their computers compromised and data stolen."
'Increasing identity theft'
And the head of the unit said: "Identity theft performed by computer viruses is on the increase.
"As in this case, it often goes on without the knowledge of the computer's owner."
Victims who receive an email from the Met should contact the detectives named within the message using the contact number provided.
Computer users are advised to regularly update their antivirus program virus signatures and scan their computers to help avoid becoming a victim.