The Information Commissioner has said it is not necessary to inform all victims of data breaches when appearing before the Leveson Inquiry today.

Christopher Graham was asked about a letter sent to the Information Commissioner's Office by Hacked Off requesting that people mentioned in the Operation Motorman files, the ICO's investigation into PI Steve Whittamore, are notified that their personal information may have been obtained illegally.

He told the inquiry: "I would have to take on a veritable army of extra people [to do this]. I'm also going to say I don't think it's necessary."

If you said to me I'd have to notify everyone named in the Motorman files, I'd be hardpressed to do so. I think [former commissioner] Richard Thomas put the point very well when he said if having established the identity of the individual and their address, we wrote to them and to say your details appear in the Motorman files, but we can't tell you why, that might be an even greater breach."

It would be a phenomenal undertaking. There are an awful lot of names and in most cases that just isn't possible."

He said the Motorman files were not always clear, referring to many named individuals who are not celebrities, but said he was "very ready" for subject access requests from those concerned.

He later added: "If Hacked Off and their lawyers are representing particular individuals then that's what we're here for. Subject access requests, here we go."

The Hacked Off campaign wrote to the commissioner yesterday.

The letter stated: "We are writing to ask you to take steps to notify those who were subjects of data-mining by the national press where you are in possession of such knowledge by virtue of the information that you obtained in Operation Motorman. "Such individuals should be told that information had been sought on them, possibly illegally; what information that was; and who (newspaper and journalist) procured it."

It is likely, if not inevitable, that media organisations have illegally retained and processed data which the ICO has judged to have been illegally obtained."

The campaign believes that individuals, many of whom do not realise they have been targeted, should be able to challenge the public interest in accessing personal information. The letter says the ICO has a positive obligation to assist people in protecting their privacy.

The inquiry heard on January 12 2012 that Express Newspapers had continued to use Whittamore until 2010, when several representatives of the group, including proprietor Richard Desmond, gave evidence. The private investigator was convicted of breaching data laws in 2004.

Robert Jay QC, counsel to the inquiry, asked the commissioner why he had not served notices on search agencies, including JJ Services, the company run by Whittamore, to determine whether the Data Protection Act was being breached.

Jay said: "We don't know one way or the other...[the press] themselves accept they don't know one way or the other. You are the regulator - you have the power to find out."

Lord Justice Leveson added: "The absence of evidence does not mean that something isn't going on."

Graham said he had to prioritise his resources and focus on "current problems where there is evidence of abuse".

He added: "I haven't asked [my investigators] to drop everything and go and see how Mr Whittamore is getting on."