Sir Richard Wilson, the cabinet secretary, told MPs yesterday that legislation was needed on special advisers.

He said a bill setting out exactly what advisers can and cannot ask civil servants to do would clarify confusion over their role.

The civil service chief told the public administration committee that the Jo Moore affair had "weakened the public trust about what goes on inside government".

But he defended Moore saying she was the victim of a whistleblower who had "breached the civil service ethos".