Rebekah Brooks asked Times editor James Harding to reveal his sources on the Milly Dowler phone-hacking story, a court has heard.
Brooks emailed the Harding on 5 July 2011 to ask how reporter Sean O’Neill knew that the News of the World had intercepted Dowler’s voice messages.
This followed an exclusive story in the Guardian that wrongly claimed that News of the World journalists had deleted messages from Dowler’s account.
Jurors at the Old Bailey were shown the email from Brooks to Harding following that morning's story in the Times. .
Brooks wrote: "We have zero visibility on the veracity of these allegations.
"Clearly Sean would have checked it out that indeed police have evidence of this.
"MPS (Metropolitan Police Service) have never mentioned it or indeed Surrey police.
"This is a proper Guardian/BBC/old Labour hit."
Brooks told Mr Harding that Labour MP Tom Watson had tweeted the day before that "exciting things were coming".
"I just need to know who gave Sean the nod it was true," she went on.
"The Yard? Surrey police? We are writing to everyone this morning.
"Our suspicion is Milly Dowler's name and address in Mulcaire's (private investigator Glenn Mulcaire) book but no evidence of actual hacking. Guardian have mixed up the Surrey police phone issues at the time and put two things together.
"But we just need to know."
The Metropolitan Police launched Operation Kilo in July 2011 track down the source of leaks from within the Operation Weeting investigation, which had been tasked to investigate allegations of phone-hacking.
It followed a complaint from lawyers representing former News of the World editor Andy Coulson, who raised concerns that details of his imminent arrest had been leaked to the Guardian.
Detective Superintendent Mark Mitchell, who led Operation Kilo, told the court that anti-corruption officers uncovered evidence showing information was being leaked by Detective Constable Peter Cripps to Guardian journalist Amelia Hill.
He told the court Cripps, who was employed to organise interviews for Operation Weeting, informed Hill about when arrests were taking place, who was being arrested and conditions of bail.
"By the nature of his role, he was effectively in possession of quite a lot of information," Mitchell said.
Cripps and Hill were both arrested but the Crown Prosecution Service (CPS) decided not to pursue charges because there was no evidence that payments had been involved, the court heard.
Brooks, 44, of Churchill, Oxfordshire, is accused of conspiring to hack phones between 3 October 2000 and 9 August 2006, along with Coulson and News of the World managing editor Stuart Kuttner.
Former News of the World and Sun editor Brooks also faces two counts of conspiring with others to commit misconduct in public office – one between 1 January 2004 and 31 January 2012, and the other between 9 February 2006 and 16 October 2008 – linked to alleged inappropriate payments to public officials.
She also faces two allegations of conspiracy to pervert the course of justice – one with her former personal assistant Cheryl Carter, 49, of Chelmsford, Essex, between 6-9 July 2011, and a second with her husband, Charles Brooks, and former News International head of security Mark Hanna and others between 15 July and 19 July 2011.
Coulson, 46, of Charing, Kent, is also facing two allegations that he conspired with former NotW royal editor Clive Goodman, 56, from Addlestone in Surrey, and other unknown people to commit misconduct in public office between 31 August 2002 and 31 January 2003, and between 31 January and 3 June 2005.
All the defendants deny the charges against them.
The trial continues.