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.

The news that the Sun newspaper obtained details of the medical condition of Gordon Brown's disabled son, Fraser, is likely to confirm the worst suspicions of the Brown entourage that News International has long been morally corrupt, but also determined to destroy him politically.

Ever since the Sun dramatically withdrew its support from Labour in September 2009, Brown has no doubt felt the paper not just betrayed him, but killed his premiership. Like Tony Blair, he had done his best to cultivate good relations with the Murdoch executives, just as he had worked hard over the years to persuade Paul Dacre at the Daily Mail that he was a moral and serious figure.

The loss of the Sun's support mattered less for its editorial comment than for the way in which the paper then slanted its daily coverage, for instance, pursuing Brown for letting down British troops in Afghanistan.

But it would seem the loss of trust between Brown and News International preceded the Sun's defection. Two months earlier, after Guardian revelations about phone hacking and the mounting evidence of a News International cover-up, Brown started to agitate for a judicial inquiry. For at least a fortnight he was in discussion with the home secretary, Alan Johnson. Brown and Lord Mandelson held discussions with Alan Rusbridger, editor-in-chief of the Guardian, to get a clearer understanding of the scandal.

Labour says Sir Gus O'Donnell, head of the civil service, blocked the inquiry. The civil service says O'Donnell resisted Brown's idea on the basis that it would be drawing the judiciary into a political process less than a year before a general election. Johnson also found himself hemmed in by civil servants, so he looked at whether an independent investigation could be launched into the original police investigation. The plan for an inquiry fell away.

After the election Brown continued to be concerned by phone hacking, encouraging lines of inquiry, firing off emails and closely following the course of a New York Times investigation into the scandal, that was finally published in September 2010.

It was around then that Brown wrote privately to the Metropolitan Police to ask whether his phone had been hacked. His suspicion was that his phone had been hacked between 2005 and 2007 when he was chancellor. By January this year, as the fact of his inquiry became known, the Met had not replied.

Some observers, even former cabinet colleagues, thought Brown's concern was the result of him still coming to terms with his electoral defeat. But it would appear Brown was convinced that, but for the opposition of News International, he might have been able to garner enough votes to form a viable coalition with the Liberal Democrats. If he had launched the judicial inquiry while in office, perhaps he could have exposed his tormentors.

That view might have been fuelled by one of his closest allies, Tom Watson, Labour MP for West Bromwich East, one of the politicians who has stuck to the News International story most doggedly. Indeed, some wrongly believe Watson's pursuit of this issue is solely driven by his loyalty to Brown.

Watson quit his job as a defence minister in 2006 as part of a battle to dethrone Tony Blair and install Brown. He believes those actions made him an enemy of Rebekah Brooks, then editor of the Sun, and a supporter of Tony Blair.

In a speech to the annual conference of the GMB last month, Watson recalled: "I was told then that Rebekah Brooks, then the editor of the Sun, now the chief executive of News International, would never forgive me for what I did to her Tony. They said she would pursue me for the rest of my life.

"They have; I can tell you from personal experience it's not very nice."

Watson rejoined the government, but in April 2009 the Sun wrongly accused him of being involved in an operation run by Damian McBride, Brown's spin doctor, to smear Conservatives. The Sun refused to withdraw its allegation and, around the same time, strangers started to look through the bins at Watson's home.

He said: "When the neighbours complained that this time their bins had also been gone through, my family was at breaking point. When our three-year-old hid behind the sofa because there was another nasty man at the door, I snapped."

In June 2009 Watson quit the government A week before he attended his first select committee hearing on 14 July that year, the Guardian published its story that News International had paid more than £1m to victims of phone hacking. Watson, already in litigation against the Sun, suddenly found himself drawn into a direct battle with News International.

His first committee taking evidence was with executives from News International, the organisation he was suing. He faced a choice. He explained: "When you're faced with that daily fear, you really only have two choices: give in and get out, or give as good as you get."

At that first evidence session, Tom Crone, legal manager of News Group Newspapers, the NI subsidiary that published the News of the World and the Sun, tried to prevent Watson sitting on the inquiry because he was suing NI. Parliamentary legal counsel told Crone that Watson would not be removed. In October 2009 the Sun was forced to accept in the high court that its story about Watson had been wrong.

From there on, Watson decided he had a cause.. He said: "I stood up in parliament and, for the first time, I told the truth that dare not speak its name – that we were scared, that the whole of British politics had been terrified into silence, become complicit in a cover-up of the illegal methods of a corporate beast that was out of control."

As the scale of the intrusion into Brown's privacy becomes known, Watson looks vindicated.