From May 25, websites will have to gain consent from web users to store or access information on their computers, under changes to European law.

The changes to the EU Privacy and Electronic Communications Directive are an attempt to protect privacy. They will require firms to tell web users what information is being stored, and explain the concept of behavioural advertising or adverts tailored to web users based on their browsing activities. As a result, web users are expected to see more pop-ups containing this information when surfing websites.

While cookies that allow web users to store goods in an online shopping basket are exempt from the changes in the directive, those cookies that allow users to store log-in details for site such as social networks or webmail are affected.

The Department for Culture, Media and Sport (DCMS), which is responsible for drawing up the steps firms will have to go through to comply with the new legislation, said its work on the regulations was "ongoing".

"Businesses need to be working to address the way they use cookies. We recognise that work will not be complete by the implementation deadline. The government is clear that it will take time for meaningful solutions to be developed, evaluated and rolled out," said Ed Vaizey, minister for culture, communications and the creative industries.

"This could cause uncertainty for businesses and consumers. Therefore we do not expect the ICO to take enforcement action in the short term against businesses and organisations as they work out how to address their use of cookies."

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See also: Mozilla to add 'do not track' feature to Firefox