Hackers have stolen an estimated £20 million from UK bank accounts, according to the National Crime Agency (NCA).

Cyber attacks in the UK

Cyber attacks in the UK

Dridex malicious software (malware) has been used in cyber attacks to collect online banking details and infiltrate accounts across the country.

"This is a particularly virulent form of malware and we have been working with our international law enforcement partners, as well as key partners from industry, to mitigate the damage it causes," said Mike Hulett from the NCA.

Britain's top crime agency has warned internet users that computers become infected with the virus when documents in legitimate-looking emails are received and opened.

"Those who commit cyber crime are very often highly-skilled and can be operating from different countries and continents," said executive assistant director of the FBI, Robert Anderson.

"We urge all internet users to take action and update your operating system. Ensure you have up-to-date security software and think twice before clicking on links or attachments in unsolicited emails."

Thousands of computers in the UK are thought to have been affected, but at least one "significant arrest" has already been made by the NCA in partnership with a US operation headed by the FBI.

"Cyber criminals often reach across international borders, but this operation demonstrates our determination to shut them down no matter where they are," said Mr Anderson.

"The criminal charges announced today would not have been possible without the cooperation of our partners in international law enforcement and private sector. We continue to strengthen those relationships and find innovative ways to counter cyber criminals."