The information commissioner Christopher Graham has responded to calls from campaign group Hacked Off for his office to notify those targeted by private investigator Steve Whittamore, the subject of the commissioner's 2005 Operation Motorman investigation.

The six-month Motorman investigation led to a raid of the Hampshire home of Whittamore and identified around 17,000 requests from the press for information from the detective, who was jailed in 2005 for illegally accessing personal data.

Graham told the Leveson inquiry this morning that the documents obtained from Whittamore's home were often "deeply obscure" and said it would be "a phenomenal undertaking" to notify the victims.

"If you said to me I'd have to notify everyone named in the Motorman files, I'd be hardpressed to do that. It isn't just a question of resources. It isn't immediately clear who is being referred to. It isn't always a celebrity, sometimes it is just a name.

"If having established the identity of an individual, we wrote to them to say 'your details appear in the Motorman report' that might be an even greater breach than the original offence. Another member of the family might see that and ask, 'what's going on?'.

"I can't tell them any more than a name appears on a file. Just because it says John Smith on a document ... I've got to work out which John Smith. It would be a phenomenal undertaking. There are an awful lot of names and in most cases that just isn't possible."

Graham left the door open to possible victims in one respect, saying that the ICO was happy to receive "subject access requests" from individuals, meaning that possible victims would have to proactively approach the ICO.

Martin Moore – director of the Media Standards Trust, which is behind the Hacked Off campaign – told Journalism.co.uk there "appears to be more than a slight contradiction" in Graham's response.

"He said he is more than happy to receive subject access requests from individuals, but if you aren't notified you were a victim how are you supposed to know? There are an awful lot of people in those files who would have no reason to believe they are listed.

"One of the things we said in our letter was that the newspaper groups were invited to see all the files, and therefore will have seen lots of the names of the victims.

"It seems rather imbalanced to us, to say the least, to give newspapers access without giving it to the victims.

Moore said that he hoped a current judicial review of the police's decision not to inform many possible victims of phone-hacking would "change the context" of the information commissioner's response this morning.

"If that finds that the police should have notified more victims it will put more obligation on the ICO to explain why it is not.

"We will continue to try and work out why on earth they should be told and the victims in the Motorman case should not."

In the Hacked Off's campaign's letter to the ICO published earlier today, Dr Evan Harris, a former MP and member of the campaign, said that victims of Whittamore identified by the operation "should be told that information had been sought on them, possibly illegally; what information that was; and who (newspaper and journalist) procured it".

Were the victims identified by Operation Motorman to be named, the newspaper groups responsible – a large number of which were identified, as opposed to only News International having been shown to have illegally hacked phones – could face a raft of civil cases.

Moore pointed out that it was the civil cases in the phone-hacking scandal "that led to the exposure of what we now know about what happened", adding that he hoped a similar process of revelation could happen in the Motorman report.

Hacked Off's request for the names of journalists, as well as victims, who used Whittamore's services to be made available comes a week after the ICO refused a similar freedom of information request from an unknown party.

Responding to the request, the ICO ruled that the names of the journalists were exempt from being disclosed under section 40(2) of the Act, the "third party personal data" exemption.

The commissioner also ruled that the names were protected by section 44(1), on "prohibition of disclosure", which covers circumstances in which disclosure may constitute contempt of court.

According to the ICO report that followed Operation Motorman, "What Price Privacy?" [PDF], there were 305 journalists named in the documents seized from Whittamore's home.

"The primary documentation seized at the premises of the Hampshire private detective consisted largely of correspondence (reports, invoices, settlement of bills etc) between the detective and many of the better-known national newspapers - tabloid and broadsheet - and magazines.

"In almost every case, the individual journalist seeking the information was named, and invoices and payment slips identified leading media groups. Some of these even referred explicitly to ‘confidential information’".

"The secondary documentation seized at the same premises consisted of the detective’s own hand-written personal notes and a record of work carried out, about whom and for whom. This mass of evidence documented literally thousands of section 55 offences, and added many more identifiable reporters supplied with information, bringing the total to some 305 named journalists."

There is no way of determining what percentage of of the requests to Whittamore identified by Operation Motorman broke the law by breaching section 55, which refers to the part of the Data Protection Act that covers unlawful obtaining of personal data. Those that were in breach of section 55 could also technically be protected by a public interest defence.

But, Hacked Off claims in it's letter, "various press organisations, for example Trinity Mirror Group, have accepted the ICO's view that many, if not most, of these transactions were illegal in that there was no public interest defence and that the data sought was of a nature that could only be obtained unlawfully".

Tina Weaver, editor of Trinity Mirror title the Sunday Mirror, told the Leveson inquiry during her evidence that "it would be surprising" if the newspaper's uses of Whittamore had all been legal.

It was also revealed at the inquiry recently that the Daily Express had used the private investigator's services as late as 2010, despite his criminal record for illegally obtaining information, and that the Mail on Sunday continued to use his services after his first arrest in 2003.

Free daily newsletter

If you like our news and feature articles, you can sign up to receive our free daily (Mon-Fri) email newsletter (mobile friendly).