Mark Thompson, director general of the BBC, has waded into the row engulfing the Metropolitan police’s attempts to force the guardian to reveal its journalistic sources signify a “dangerous period” for British journalism.

Despite later sropping their legal moves the Met’s decision is being seen as part of a “disturbing trend” which has seen police forces seeking sources and material.

Thompson said: “At the BBC, we receive an ever-growing number of demands for untransmitted news rushes which the police seem to regard as having no more privilege or protection attached to them than CCTV pictures.”

A court order is required for the release of such material.

Thompson also warned that “It would be easy for concern over the appalling invasions of privacy revealed by the phone-hacking scandal to spill over into legislation or regulation which enables wrongdoers to escape journalistic exposure.”