Mr Campbell, who served as spokesman, press secretary and communications director under Tony Blair when he was prime minister, gave evidence in London on Tuesday - day 10 of the trial of damages claims brought by the Duke of Sussex and other high-profile figures against Mirror Group Newspapers (MGN) over alleged unlawful information gathering at its titles.
MGN, which also publishes the Sunday Mirror and Sunday People, is contesting the claims over allegations its journalists were linked to voicemail interception, securing information through deception and hiring private investigators for unlawful activities.
Mr Campbell, who was a journalist for the Daily Mirror and Sunday Mirror before becoming a spin doctor, said in a witness statement he had been shown three documents by the claimants’ legal team, dating from when he was chief press secretary in Downing Street.
Two were invoices from private investigation firm Southern Investigations, both addressed to then-Daily Mirror investigations editor Gary Jones, and the third was a confidential hand-written note which contained numbers for financial accounts held by him and his wife Fiona Millar, he said.
His statement read: “I find the fact that MGN unlawfully obtained such private and personal financial information about myself and Fiona extremely troubling, especially because of my role in government at that time.
“The fact that they did not publish the information they obtained in no way minimises this view.
“I was, of course, very aware that the media viewed me as someone of public interest.
“Having been a journalist myself, and also having been briefed regularly on issues of personal and government security, I was very careful in the protection of information that was confidential within government or private to me and my family.
“Though our mortgage details were not published, it leaves us wondering what other stories about us may have been obtained illegally.”
Mr Campbell said he believed the inquiries into his affairs related to a story involving Labour grandee Peter Mandelson’s finances which was published on Christmas Eve 1998.
He said he had also seen documents which indicated that a “blagger” sent Lord Mandelson’s private banking and mortgage details to Southern Investigations, who sent them on to Gary Jones, now editor of the Daily Express, on December 23 of that year.
Mr Campbell said in his statement: “As a result, I believe that the Daily Mirror decided, at the time of the Mandelson story, to fish into my bank and mortgage affairs in the hope that they too would reveal something they considered newsworthy.
“I would add that the publishing of Peter Mandelson’s mortgage details led not only to a public row and his resignation, but also provoked considerable recriminations within government, as people made assumptions about who had put this information into the public domain, and to what end.
“This blame game was further damaging to the government, and now knowing how the information actually came to be made public, adds to my disgust at the methods used.”
Mr Campbell also accused former Daily Mirror editor Piers Morgan of “two-faced conduct” and said he did not believe Mr Morgan would not have been aware of such conduct.
He said in his statement: “I have read the section of Piers Morgan’s memoirs, The Insider, dealing with late December 1998 and I can now see that Mr Morgan does not set out truthfully how he obtained the information on Mr Mandelson.
“At the same time that I now know he was commissioning or approving Southern Investigations, through Mr Jones, to break in to my bank account, his book shows that he was seeking to ingratiate himself with the prime minister and myself at Downing Street, and pretending to sympathise about what had happened to Peter Mandelson.
“Mr Morgan’s two-faced conduct, in purporting to be a real ally of the prime minister and the Labour government, while all the time he and his senior team were using illegal means to find stories designed to destabilise that government, compounds the anger I feel about this, as does the fact that this conduct has been emphatically denied, by Mr Morgan and his colleagues, for so long.
“Fiona and I are shocked and, frankly, appalled at this intrusion into our privacy by the Daily Mirror.
“I find it very hard to believe that any editor, especially one as hands-on as Mr Morgan, would not have known and demanded to know where the big stories were coming from.
“Nor do I believe that people in senior positions in government with access to highly sensitive information and with obvious security concerns would have been targeted in this way without the editor knowing and sanctioning such methods.”
Under cross-examination by MGN’s barrister Andrew Green KC, Mr Campbell confirmed he has never previously claimed to have personally witnessed phone hacking or other unlawful information-gathering while working at the Daily Mirror and Sunday Mirror.
“I don’t believe believe any of the editors that I worked for would have sanctioned it,” he said.
Mr Campbell accepted that the 1998 “scandal” story on Lord Mandelson’s finances was “a matter of public interest”.
Mr Green told the court he was “not for one minute” suggesting that Mr Campbell was suspected of being involved in wrongdoing.
Mr Campbell faced questions over the confidential source of a document, allegedly seized by police after a raid on Southern Investigations in 1999, which the former No 10 press secretary says contains his financial information.
“I trust the person, my solicitor, who brought this to me. I have no reason to question and don’t question the provenance thereof,” Mr Campbell said.
The duke is one of four “representative” claimants whose cases have been selected for a trial, being heard over six to seven weeks in London.
Coronation Street actors Michael Turner and Nikki Sanderson and comedian Paul Whitehouse’s ex-wife Fiona Wightman are also named as “representative” claimants.
MGN is contesting the claims over allegations its journalists were linked to voicemail interception, securing information through deception and hiring private investigators for unlawful activities.
MGN says board members have denied knowledge of such activities and claims there is “no evidence, or no sufficient evidence, of voicemail interception” in any of the four claims chosen as “representative” cases.
The hearing before Mr Justice Fancourt continues.