News of the World editor Andy Coulson convinced his deputy, from west London, that a scoop about then home secretary David Blunkett's affair with a married woman had come from a "source" – when in fact it was from phone-hacking, a court has heard.
Neil Wallis, Coulson's second-in-command between 2003 and 2007, is accused of being part of the hacking plot which prompted the Sunday tabloid's closure in 2011.
His Old Bailey trial has heard that, in the summer of 2004, private investigator Glenn Mulcaire accessed the voicemails of Blunkett's lover, leading to a front-page splash on 15 August.
The prosecution have asserted that, despite being on holiday at the time, Wallis must have known what was going on, given that Coulson and others have already been convicted of being part of the conspiracy.
But giving evidence in his defence, Wallis named his ex-boss as the one who told him the story originated from a well-placed source.
He told the court that Coulson had briefed him in early August about chief reporter Neville Thurlbeck re-starting his investigation into Blunkett.
"Andy called me in and said he was going to revisit Thurlbeck's Blunkett investigation. We had talked about it. There were a lot of issues about this in terms of practicality with the politics."
Wallis told jurors he wanted to make sure the reporter "did not mess this up", adding: "This was a guy who was surrounded around the clock by armed police officers.
"Following him by definition is going to be a very dangerous, never mind difficult, thing to do without getting caught – or worse. One of our journalists could have got shot."
He recalled having a conversation with Thurlbeck about the story, telling him '"Don't get caught, it would be really embarrassing for the paper".
On 8 August, Wallis went to France with his wife, who had been suffering from health problems, but records showed he still had phone conversations with Coulson.
He told the court he made an eight-minute call to his boss on 12 August to "unburden" himself after his wife had a crisis, but they would "inevitably" have talked about what was going on at the paper too.
The following day, Coulson rang him en route to Blunkett's constituency office in Sheffield, the court heard.
Describing the six-minute call, Wallis said: "He called me that morning first thing. He told me what he was going to do. He said he was on his way to Yorkshire to confront – put the story to – David Blunkett."
Asked by his lawyer, Neil Saunders, where he thought the story had come from, he said: "I honestly thought it had come from sources close to [the married woman]."
When quizzed about who had told him that, he said: "I think, Andy. I could not swear to that."
Wallis told the jury he was "taken aback" by his boss's plan, but he did not let on, saying: "What difference would it make at that stage?
"He was the boss and he had decided to do it. He was on the motorway near Sheffield and he was virtually there. He was not going to change his mind now so I was not going to put doubt in his mind."
Asked if this was an unusual state of affairs, he said: "Confronting a home secretary about an alleged affair with a married woman? I can't think of another example."
In 2011, police found five micro-cassettes of recordings from the married woman's voicemails in the safe of a NoW lawyer, the court has heard.
Wallis, 64, of Chiswick, denies conspiracy to hack phones.
The trial continues.