David Cameron "probably" rode a horse which was lent by police to former News of the World editor Rebekah Brooks, his spokeswoman said.
The Prime Minister rode with Mrs Brooks's racehorse trainer husband Charlie, a friend since they went to school at Eton, while he was Leader of the Opposition.
His mount may have been Raisa, the ex-Scotland Yard horse that it emerged this week was stabled at the couple's Cotswolds home from 2008-10, the spokeswoman said.
But he had "no recollection" of ever having ridden with the ex-tabloid editor herself and had not ridden with Mr Brooks since entering 10 Downing Street, she insisted.
Asked about the connection in an interview with Channel 5 News, Mr Cameron said: "It's a matter of record that I have been riding with Rebekah Brooks's husband, Charlie Brooks.
"He is a friend of mine for 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."
The spokeswoman declined to disclose the identity of the horse to which he was referring.
Press Association, photo by The Prime Minister's office