In Delivering inclusive websites:

In Guidance:

The minimum standard of accessibility for all public sector websites is Level Double-A of the W3C Web Content Accessibility Guidelines. All new websites must conform to these guidelines from the point of publication.
Websites owned by central government departments must be Double-A conformant by December 2009. This includes websites due to converge on Directgov or BusinessLink, unless convergence is scheduled before this date.
Websites owned by central government executive agencies and non-departmental public bodies must conform by March 2011.
Government websites owners are reminded to follow the conditions of use for a .gov.uk name (Registering .gov.uk domain names (TG114)). Websites which fail to meet the .gov.uk accessibility requirements may be at risk of having their domain name withdrawn.
Compliance with the Web Content Accessibility Guidelines is acceptable at Level Double-A of version 1.0 or the equivalent level in version 2.0. Future policy and timetables for implementation will align with European Commission recommendations on the adoption of WCAG 2.0. Planned future updates to this guidance will include details of the specific conformance requirements for version 2.0.

Content formats not covered by version 1.0 of the Web Content Accessibility Guidelines (e.g. Flash, PDF, JavaScript etc) should only be used if it is determined that they are the most appropriate for the intended purpose. For example, this could be where the proposed content enhances the functionality or understanding for the intended audience.
Every attempt should be made to ensure that the accessibility features of the relevant authoring tool are used.
In all cases, web content must always be developed according to the process set out in your website accessibility policy.