London's Metropolitan Police Computer Crime Unit (CCU) has launched an investigation into how data containing personal information from the UK ended up on a computer in the US.
"We believe the data has been stolen with the use of a computer virus, and it is believed that more than 2,300 compromised computers in the UK consisting of 83,000 files have been targeted," the unit said in a statement.
Email addresses and other confidential data have been recovered, including passwords, credit card numbers and information relating to online transactions.
The CCU said that the information had been harvested from the computers by a type of malicious code. It is currently involved in contacting victims of the hacking, and alerting ISPs, banks and online e-commerce services.
The CCU has appealed to web users to look out for its emails and not to treat them as spam or phishing attacks.
"We are contacting victims via email. If you receive one of these emails, it will have an email address of 'met.police.uk'. Please contact the detectives named within the email on the contact number given," said the CCU.
"You will not be asked to disclose any passwords and the email outlines the specific security instructions to follow."
The Met also advises users who make financial transactions online to regularly check bank statements.
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