Apology – the Sun: the original version of the article below incorrectly reported that the Sun newspaper had obtained information on the medical condition of Gordon Brown's son from his medical records. In fact the information came from a different source and the Guardian apologises for its error

Journalists from across News International repeatedly targeted the former prime minister Gordon Brown, attempting to access his voicemail and obtaining information from his bank account and legal file as well.

There is also evidence that a private investigator used a serving police officer to trawl the police national computer for information about him. That investigator also targeted another Labour MP who was the subject of hostile inquiries by the News of the World, but it is not confirmed whether News International was specifically involved in trawling police computers for information on Brown.

Separately, Brown's tax paperwork was taken from his accountant's office apparently by hacking into the firm's computer. This was passed to another newspaper.

Brown was targeted during a period of more than 10 years, both as chancellor of the exchequer and as prime minister. Some of the activity clearly was illegal. Other incidents breached his privacy but not the law. An investigation by the Guardian has found that:

• Scotland Yard has discovered references to Brown and his wife, Sarah, in paperwork seized from Glenn Mulcaire, the private investigator who specialised in phone hacking for the News of the World.

• Abbey National bank found evidence suggesting that a "blagger" acting for the Sunday Times on six occasions posed as Brown and gained details from his account.

• London lawyers Allen & Overy were tricked into handing over details from his file by a conman working for the Sunday Times.

• Details about his infant son's medical condition were obtained by the Sun, which published a story about the child's serious illness.

Brown joins a long list of Labour politicians who are known to have been targeted by private investigators working for News International, including the former prime minister Tony Blair and his media adviser Alastair Campbell, the former deputy prime minister John Prescott and his political adviser Joan Hammell, Peter Mandelson as trade secretary, Jack Straw and David Blunkett as home secretaries, Tessa Jowell as media secretary and her special adviser Bill Bush, and Chris Bryant as minister for Europe.

The sheer scale of the data assault on Brown is unusual, with evidence of "attempts" to obtain his legal, financial, tax, and police records as well as to listen to his voicemail. All of these incidents are linked to media organisations. In many cases, there is evidence of a link to News International.

Scotland Yard recently wrote separately to Brown and to his wife to tell them that their details had been found in evidence collected by Operation Weeting, the special inquiry into phone hacking at the News of the World. It is believed that this refers to handwritten notes kept by Mulcaire, which were seized by police in August 2006 and never previously investigated. Brown last year asked Scotland Yard if there was evidence he had been targeted by the private investigator and was told there was none.

Journalists who have worked at News International say they believe Brown's personal bank account was accessed on several occasions while he was chancellor. An internal inquiry by Abbey National's fraud department found that during January 2000 someone acting on behalf of the Sunday Times contacted their Bradford call centre six times, posing as Brown, and succeeded in extracting details from his account.

Abbey National's senior lawyer sent a summary of their findings to the editor of the Sunday Times, John Witherow, concluding: "On the basis of these facts and inquiries, I am drawn to the conclusion that someone from the Sunday Times or acting on its behalf has masqueraded as Mr Brown for the purpose of obtaining information from Abbey National by deception."

Abbey National were not able to identify the bogus caller who tricked their staff. It is a matter of public record that a Sunday Times reporter frequently used the services of a former actor, John Ford, who specialised in "blagging" confidential data from banks, phone companies and the Inland Revenue (now HM Revenue & Customs).

Also in January 2000, one of the paper's reporters used a conman named Barry Beardall, who was subsequently jailed for fraud, to trick staff at Allen & Overy into handing over details from his personal file.

A tape made by Beardall at the time reveals that he claimed to be an accountant from the "Dealson group of companies" and that they were interested in buying Brown's flat. Beardall also practised trickery in an attempt to provide Sunday Times stories about Blair, when he was prime minister, and Labour's candidate for the mayor of London, Frank Dobson.

Confidential health information for Brown's family have reached the media on two different occasions. In October 2006, the then editor of the Sun, Rebekah Brooks, contacted the Browns to tell them that the paper had learned that their four-month-old son Fraser had been diagnosed with cystic fibrosis.

This appears to have been a clear breach of the Data Protection Act, which would allow such a disclosure only if it were in the public interest. Friends of the Browns say the call caused them immense distress, since they were only coming to terms with the diagnosis, which had not been confirmed. The Sun published the story.

David Muir, one of Brown's most senior advisers at No 10, said: "They were contacted by Rebekah Brooks, who told them that they had information that Fraser had cystic fibrosis, which was a matter that they, the family, were just getting their heads around at the time and dealing with. They didn't know how Rebekah came across this information and now, what's come to light, it was obtained by what appeared to be illegal methods."

Brown's spokeswoman said: "The family has been shocked by the level of criminality and the unethical means by which personal details have been obtained. The matter is in police hands."

When their first child, Jennifer, was born on 28 December 2001, a small group of specialist doctors and nurses was aware that she had suffered a brain haemorrhage and was dying. By some means which has not been discovered, this highly sensitive information was obtained by news organisations, who published it over the weekend before Jennifer died, on Monday 6 January 2002.

In 2003, Devon and Cornwall police discovered that one of their junior officers was providing information from the police national computer to a network of private investigators. The Guardian has established that one of these investigators, Glen Lawson of Abbey Investigations in Newcastle upon Tyne, used this contact to commission a search of police records for information about Brown on 16 November 2000. Lawson also commissioned searches related to two other Labour MPs, Nick Brown and Martin Salter.

Lawson made these searches on behalf of journalists, a previously unreported court hearing was told. Transcripts obtained by the Guardian show that the search on Martin Salter was made at a time when the News of the World, then edited by Brooks, was attacking him for refusing to support the paper's notorious "Sarah's law" campaign to name paedophiles. Lawson currently refuses to name the journalists who commissioned him.

An attempt to prosecute this network was blocked by a West Country judge, Paul Darlow, who ruled it would be a misuse of public money to pursue the case. However, Devon and Cornwall police contacted Brown's office to warn him that he had been a victim, as they also did with his two Labour colleagues.

Brown's tax paperwork was obtained from the offices of his accountants, Auerbach Hope, in late 1998.

The first sign that the records had been taken came when a journalist from the now defunct Sunday Business called the accountants to say that they had been passed a copy of the records, including a schedule of Brown's income for the most recent year. The journalist acknowledged that the paperwork showed no sign of any kind of wrongdoing on Brown's part but wanted to do a story about the fact that it had been stolen.

Police came and found no sign of any break-in. The originals of the documents were still in Brown's file, which ruled out the possibility that they had been taken from the firm's dustbins.

Auerbach Hope discounted theft by an insider on the grounds that they would have stolen paperwork which showed wrongdoing and thus had greater media value. They concluded that the most likely explanation was that somebody had hacked into their computer systems, specifically targeting Gordon Brown.

Senior Labour figures also strongly suspect that a news organisation broke the law to obtain the emails that led to the resignation in April 2009 of Brown's close aide Damian McBride. The emails, which disclosed a scheme to smear Tory MPs, had been exchanged between McBride and a Labour party activist, Derek Draper. The Labour figures believe that the emails were hacked from Draper's computer and that their contents were then sent to the political blogger Guido Fawkes, whose stories were then followed by Fleet Street.News International said in a statement: "We note the allegations made today concerning the reporting of matters relating to Gordon Brown. So that we can investigate these matters further, we ask that all information concerning these allegations is provided to us."