P&EC "cookie" Regulations
Privacy and Electronic Communications (EC Directive) (Amendment) Regulations SI 2011/1208
From 26 May 2011, you must obtain the informed consent of users, visitors and/or subscribers to your website before you install cookies, or other means of storing information, on their terminal equipment or mobile devices.
- the type of data files being placed on users' equipment and whether or not they are essential
Government advice also available at the ICO advice page