The government currently uses a scheme of ‘protective markings’ to classify the sensitivity of the information and documents it holds. The protective markings are, in ascending order of security, protect; restricted; confidential; secret; top secret. For further information see the HMG Security Policy Framework.
As of 2 April 2014 protective markings will be abolished and replaced by ‘security classifications’. Those security classifications will be ‘official’, ‘secret’, and ‘top secret’.
For further information see the document Government Security Classifications and accompanying guidance.