What is known about US spy programme Prism?

Documents leaked by former CIA worker Edward Snowden suggest that the data-gathering centre GCHQ has had access to a US internet monitoring programme since at least June 2010.

Prism is said to give the National Security Agency and FBI easy access to the systems of nine of the world's top internet companies, including Google, Facebook, Microsoft, Apple, Yahoo and Skype.

Rory Cellan-Jones reports on what is known about Prism.