The BBC's Leon Hawthorne: Greater security for the security services

The investigator will have greater access to secret files than the Intelligence and Security Committee, to which he or she will report.

The appointment follows calls by the former MI5 agent David Shayler for greater scrutiny of the secret services to prevent illegal activities and incompetence.

Mr Shayler was charged with breaking the Official Secrets Act after he made a series of claims about the working of the security services, including the allegation that members of MI6 aided an unsuccessful assassination attempt against the Libyan leader Colonel Muammar Gaddafi.

The UK government, which has denied any involvement in the assassination plot, failed last year in its bid to have Mr Shayler extradited from France.

Tom King: Some things must remain secret

Currently, the security services are allowed to withhold information from the Intelligence and Security Committee if it deems the material too sensitive.

The committee is also responsible to the prime minister - who appoints its members - rather than to Parliament.

Its chairman, Tory MP Tom King, said the new investigator would have greater powers to examine future allegations against the security services.

"There may be some things for which we are denied access because they say they are too sensitive and endanger national security - they are secrets that must be totally protected.

"In those circumstances we have said that we need an independent investigator who will have access to anything and will report back to us," he said.

Campaigners unhappy

BBC Correspondent Leon Hawthorne says the job is likely to go to somebody from the intelligence community.

John Wadham: Investigator must be independent

But John Wadham, one of Mr Shayler's lawyers and the director of the campaigning organisation Liberty, says this is not good enough.

"We want somebody who is independent and is seen to be independent. I would like to see this job advertised," he said

"Of course you couldn't take on anybody to do it, you would have to trust them. But there are many independent people who can be trusted."