Only senior judges would be able to sit on an inquiry and an appointment could not be made without the approval of the Lord Chief Justice, who would want to look into why it was required, he said.

BBC: MPs back 'secret inquests' plan

The prime minister told the inquiry he accepted hiring Mr Coulson was "a controversial appointment" which had "come back to haunt both him and me".

BBC: UK

Mr Cameron announced their appointment in a statement to the House of Commons, and was speaking after the public inquiry into Stafford Hospital where years of abuse and neglect led to the death of hundreds of patients.

BBC: Ann Clwyd asked to advise on NHS hospital complaints

In the House of Lords, the government was urged by Lord Marlesford to include in the inquiry "the possibility of replacing the top operational command of UKBA with the appointment of retired military officers who have spent their professional lives defending the realm".

BBC: Let army officers look after UK borders, peer urges

Sir Alistair's three-year appointment comes amid criticism of the way complaints against the police are handled in the aftermath of the Stephen Lawrence Inquiry.

BBC: Pledge over police complaints