Rupert Murdoch has claimed Gordon Brown rang him in an "unbalanced" state of mind and "declared war" on his media empire after The Sun switched support to the Conservatives.
The News Corporation chairman and chief executive told the Leveson Inquiry that successive British prime ministers wooed him to win his papers' backing, but insisted he never asked them for any favours.
Mr Murdoch, 81, said Tony Blair was a "personal friend" and described how David Cameron as leader of the opposition took a detour from a holiday in Turkey to meet him on his daughter's yacht off a Greek island in 2008.
At the start of a two-day appearance before the press standards inquiry, the billionaire recounted how his one-time "warm personal relationship" with Mr Brown broke down after The Sun stopped backing Labour.
Britain's top-selling newspaper announced it would support the Conservatives at the next general election on its front page on September 30 2009, the day after Mr Brown's speech to the Labour Party conference in Brighton.
Mr Murdoch told the Leveson Inquiry that the then-prime minister phoned him after this and asked him if he knew what The Sun was doing. He said he told Mr Brown he was not warned about the exact timing of the paper's announcement and added: "I am sorry to tell you Gordon, we have come to the conclusion that we will support a change of government when and if there is an election."
The media tycoon went on: "He said, 'Well, your company has declared war on my government and we have no alternative but to make war on your company'. And I said, 'I'm sorry about that Gordon, thank you for calling', and end of subject."
Mr Murdoch said he did not know how the former prime minister might have "made war" on News Corp, adding: "I don't think he was in a very balanced state of mind."
But Mr Brown said that the "serious allegation" that he declared war on the media mogul's company was "wholly wrong", and called on Mr Murdoch to correct his evidence.
Mr Murdoch also told the inquiry that Mr Brown made a "totally outrageous" statement after the phone-hacking scandal broke last July, suggesting a 2006 Sun story about his son Fraser having cystic fibrosis had been obtained illegally.