The Daily Mail spent an estimated £143,000 asking a private eye to make 1,728 potentially illegal requests to unearth phone numbers and addresses of public figures over a three-year period, including personal details of the Duchess of Cambridge and her sister Pippa Middleton.

Journalists at the newspaper asked for private information on average more than once a day, and occasionally asked for individual criminal record checks. Its reporters demanded roughly twice as many searches as was previously thought, according to research by ITV News.

The tabloid demanded the private information between 2000 and 2003 from Steve Whittamore – whose targets for a range of newspapers included the union leader Bob Crow, the family of the murder victim Holly Wells, members of the England football team and the singer Charlotte Church.

The Daily Mail made the most requests, with its sister title the Mail on Sunday spending an estimated £62,000 on 578 requests for information. The Sunday title's figure was also roughly double the number of requests counted by the information commissioner in a 2006 report.

It had previously been thought that the Daily Mail made 982 such requests, according to the information commissioner, but ITV News's examination of Whittamore's notebooks, which were seized when he was arrested in 2003, suggests that the figures were undercounted.

Journalists from the Mail obtained the BT friends and family numbers of people of interest 90 times, at an average cost of about £336. Reporters obtained 1,285 ex-directory numbers at a cost of £65 a time. There were 20 requests to establish a person's address from their vehicle registration at an average cost of £150.

On three occasions, Whittamore's JJ Services was asked for an individual's criminal records to be checked against the police national computer, each at a cost of £500.

The Observer, owned by Guardian Media Group, publishers of the Guardian, made 201 requests for information from Whittamore at a cost of £13,270 between 2000 and 2003. This was about double the 103 previously counted by the information commissioner, and the new data says the Sunday paper made 182 requests for ex-directory numbers, asked for 18 mobile and landline numbers to be linked to people's addresses or other personal details and requested one "blag".

Obtaining such personal information is a breach of section 55 of the Data Protection Act, although there is a public interest defence. If anybody working in the public sector was paid money to supply information illegally, it could amount to an offence under the more serious 1906 Prevention of Corruption Act, for which there is no public interest defence. Whittamore himself pleaded guilty to breaches of the Data Protection Act in 2005 and received a two-year conditional discharge.

Paul Dacre, the Daily Mail's editor in chief, was questioned about his title's use of Whittamore at the Leveson inquiry in February, and in particular about requests to supply a person's friends and family numbers to the newspaper. Dacre said that information obtained from Whittamore "could all be obtained legally, but it would take time. This was a quick and easy way to get that information".

On Wednesday, the publisher of the Daily Mail and the Mail on Sunday, Associated Newspapers, said the allegations were familiar from reports published by the information commissioner in 2006 in the wake of the Whittamore case – known as Operation Motorman.

In a statement, the publisher said: "Although the commissioner did not disclose details of the information obtained, it would seem, for the most part, it related to the tracing of individuals' addresses and phone numbers.

"The report recognised that many of these cases would have been covered by public interest defences. Indeed it is good practice that matters concerning individuals in the news should be put to them before publication to ensure accuracy and give the opportunity to offer comment. To do this it is vital to trace addresses and telephone numbers which in the main could be obtained through legal means."

Church said she had been shown a copy of the information Whittamore had collected on her. She said: "It was basically just kind of DVLA records, so registration numbers and house numbers and mobile numbers and criminal records if applicable. It was about literally everybody I had ever known. Anybody I had ever come into contact with. That's what took us by surprise about it. Lots of my parents' friends … some of my mum's old work colleagues … a phenomenal amount of information."

The Daily Mirror was Whittamore's second biggest customer, using the private eye 984 times between 2000 and 2003 – about 300 occasions more than previously counted by the information commissioner – and spending about £92,000. The bulk of the information sought was ex-directory numbers, but its journalists asked for number plates to be traced to individuals on 79 occasions and for criminal record checks 19 times. Its Sunday sister title, the Sunday People, was also a heavy user of Whittamore, spending an estimated £76,295 on 1,016 requests. Overall, titles owned by Trinity Mirror, publisher of the Daily Mirror, spent £200,623, while Associated Newspapers spent £234,773, a figure that also includes the Evening Standard, which the publisher owned at the time.

Trinity Mirror said the detail "was the subject of a police investigation in 2003 when no action was taken against any journalist". It added: "We took our lead from the ICO [information commissioner's office] and did what was asked of us by reaffirming to our national newspaper editors that the company's policy was to comply with the criminal law and the PCC code." News International was not a heavy user of Whittamore between 2000 and 2003, with the News of the World the biggest-spending title in the Murdoch group, spending £23,306 on 240 requests for information, according to ITV News.

A spokesperson for the Observer said Whittamore's papers were described by the ICO as "deeply obscure" and that, as a result, it was "difficult to determine exactly what he was doing. As such, the Observer has always said that it is not possible to be absolutely certain that everything he did for the paper would have met a strict definition of 'the public interest'. Therefore, since the publication of the ICO's report in 2006, we have strengthened the process by which the use of private investigators is approved. In fact, none have been approved since then."

Dr Evan Harris of the Hacked Off campaign group said that subjects of press intrusion should have been informed that they were targeted by journalists via Whittamore. He said "there been a sophisticated but flagrant cover-up of the extent and knowledge of this by several newspaper groups" and that there had been "no proper investigation by the authorities of the role of elements of the press in driving this industrial scale theft of private information".