The Prime Minister "probably" rode a horse loaned by the police to Rebekah Brooks, the former editor of the News Of The World, his spokeswoman has admitted.
David Cameron has been friends with her husband, Charlie Brooks, since their schooldays at Eton.
He confirmed he had ridden horses with Mr Brooks but has "no recollection" of riding with Mrs Brooks.
The Prime Minister's spokeswoman said the horse he borrowed may have been Raisa, the ex-Scotland Yard horse loaned to Mrs Brooks between 2008 and 2010.
"It's a matter of record that I have been riding with Rebekah Brooks' husband, Charlie Brooks," Mr Cameron told Five News.
"He is a friend of mine of 30 years' standing and a neighbour in my constituency.
"But since I have been Prime Minister I think I have been on a horse once and it wasn't that one."
Mrs Brooks, who remains on bail after being questioned by detectives on suspicion of phone hacking and corruption, quit as chief executive of News International last summer.
She asked to "foster" a horse that had been retired from active service but returned Raisa two years later in 2010.
A police spokesman said the horse was in a "poor but not serious condition" and has since died of natural causes.
Details of the horse loan emerged amid intense scrutiny of the relationship between the police and the press.
Lord Justice Leveson, who is leading a wide-ranging inquiry into press ethics, criticised continued leaks.
In what was understood to be a reference to the horse loan story, he said the revelations ran the risk of "disrupting the way in which this inquiry can proceed".
On Thursday, a former senior Met officer told the inquiry his relationship with individuals at the NOTW did not influence his decision not to launch a fresh investigation into phone hacking.
John Yates, who resigned as assistant commissioner at Scotland Yard last year, said: "I absolutely know my links to the News Of The World played no part in my decision-making... my conscience is clear."
Mr Yates confirmed he was a "good friend" of former NOTW executive editor Neil Wallis and went to football matches with him, but he said at no time did he have any improper conversations with Mr Wallis on the many occasions they were together socially.
He was also questioned about get-togethers he had with Lucy Panton, the NOTW's crime reporter.
Mr Yates admitted he "may well" have drunk champagne with Ms Panton, but denied that he did her favours in return.