A global network of hacked computers aiming to steal bank details has been halted temporarily, giving Brits two weeks to protect themselves.
The National Crime Agency (NCA), FBI and other international law enforcement agencies have disrupted malicious software called Gameover Zeus (GOZeuS) operating through a network of infected computers but the NCA said it could be as little as two weeks before the hackers create new ways to get it going again.
GOZeuS, which is also known as P2PZeuS, is responsible for the fraudulent transfer of hundreds of millions of pounds globally, according to Action Fraud.
More than 15,500 computers in the UK could be currently infected, with many more potentially at risk.
The NCA said: "Users are typically infected by clicking on attachments or links in emails which may look like they have been sent by genuine contacts and may purport to carry invoices, voicemail messages, or any file made to look innocuous. These emails are generated by other victims' computers, who do not realise they are infected, and are used to send mass emails creating more victims."
The virus only affects Windows, including Windows running on Apple Macs, and people are being urged to install anti-virus software and update their operating systems to the latest version to stop it form infecting their machines.
Users may be sent letters and emails from their internet service providers (ISPs) warning them about the scam. However, ActionFraud has warned that not all correspondence will be legitimate.
"Fraudsters are opportunistic and might send out phishing emails claiming to be from your ISP or from a law enforcement agency urging you to click on a link or download an attachment for a solution. Never click on suspicious links or download attachments from unsolicited emails," ActionFraud said in a statement.
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Andy Archibald at the NCA added: "Nobody wants their personal financial details, business information or photographs of loved ones to be stolen or held to ransom by criminals. By making use of this two-week window, huge numbers of people in the UK can stop that from happening to them.
"Whether you find online security complicated or confusing, or simply haven't thought about keeping your personal or office computers safe for a while, now is the time to take action. Our message is simple: update your operating system and make this a regular occurrence, update your security software and use it and, think twice before clicking on links or attachments in unsolicited emails."