The Metropolitan Police has admitted that during the first four years of the phone-hacking case it warned only 36 people that they may have been targeted by Glenn Mulcaire, the private investigator employed by the News Of The World.
The number now believed to have been found on the extensive list of names and telephone details held by Mulcaire, and found by detectives in 2006, is now 4,000.
But in a letter to John Whittingdale, chairman of the Commons culture, media and sport committee, the Met's acting deputy commissioner, John Yates, admitted that fewer than 1 per cent of the potential victims were contacted by officers.
The first investigation five years ago led to 28 people being warned. A further eight received warnings in 2009. Until yesterday, Scotland Yard had maintained that the numbers should be kept secret.
Having assured the Director of Public Prosecutions that it would warn all of the "potential victims", its conduct in carrying out the investigation may now be further called into question.
A new investigation was established in January, overseen by Mr Yates himself, under which the remaining people on Mulcaire's list are in the process of being contacted.
Mr Whittingdale has been a vocal critic of the Met investigation. Earlier this month, he called for a public inquiry into how the force failed to find cases to pursue among News Of The World employees beyond that of its former royal editor, Clive Goodman, who has already served a jail sentence for his part in the affair.
"There are some very big questions. What I find worrying is the apparent unwillingness of the police, who had the evidence and chose to do nothing with it.
"That's something that needs to be looked into," he told the BBC after the News Of The World published an apology to those whose voicemails had been hacked.
He added: "It also raises some quite serious questions for the security of government. It seems pretty extraordinary that newspapers are able to listen in to the private conversations of Downing Street, royal staff and others.
"I'm wanting to know through the Home Office why those responsible for safeguarding security weren't able to do anything about it."