The Sun today claims allegations that it hacked into Gordon Brown's family medical records to reveal that his son had cystic fibrosis were "false and a smear".
And the paper suggests the former Prime Minister has a grudge against the paper because it switched to supporting the Tories at the last general election.
The Sun splashes on its defence and says: "We discovered the ex-PM's four-year-old son Fraser had cystic fibrosis months after his birth. We can reveal the source of our information was a shattered dad whose own son also has the crippling disease and who wanted to highlight the plight of sufferers.
"And when we approached former Labour leader Mr Brown and his wife Sarah with the story, she gave us their consent to run it."
The Sun also claims: "The dad, whose own son has cystic fibrosis, told the Sun about Fraser because he hoped to give the crippling condition a higher public profile.
"And he believed the fact that Mr Brown's family was coming to terms with it would do so."
The paper says it has a sworn affidavit from its source and has produced a video interview with the man in which his identity is disguised.
The Sun claims David Cameron "was also embarrassed last night after he hastily jumped on the bandwagon to back the false claims."
It adds that political commentators believe Brown has a "bitter grudge" against the Sun because it switched from Labour to back Cameron and the Tories at the last election.
It says former Sunday Times editor and BBC presenter Andrew Neil claimed Brown rang Mr Murdoch at the time of the switch in October 2009 to tell him: "I will destroy you."