Labour called for David Cameron to "come clean" over close links to media bosses after he admitted riding a horse lent by Scotland Yard to ex-News of the World editor Rebekah Brooks.
The Prime Minister ended speculation by confirming that the ex-police horse, named Raisa, was among his mounts on rides with Mrs Brooks's husband Charlie - a friend from their Eton schooldays.
He apologised for allowing a "confusing picture" to emerge about his personal connection to the animal, leaving him forced to address the issue at a Brussels post-summit press conference.
"He has a number of horses and, yes, one of them was this former police horse Raisa which I did ride," he said of his outings with Mr Brooks, which he stressed were all before he became premier.
"I am very sorry to hear that Raisa is no longer with us and I think I should probably conclude by saying I don't think I will be getting back into the saddle any time soon," he quipped.
The revelation caused mirth among politicians. Labour leader Ed Miliband joked that that must have been what Mr Cameron meant when he talked of "cleaning out the stables at News International".
But the Labour leader said that, while the story was "ridiculous", it was symbolic of a more serious issue.
Speaking to Labour's Scottish conference in Dundee, Mr Miliband said: "Today we learn that Cameron cannot even manage to be straight about riding a horse loaned to him by a News International executive.
"The horse story may seem ridiculous. It is. But it is also becoming the symbol of how leading politicians got too close to powerful media players."
Mr Miliband's deputy Harriet Harman added: "David Cameron has not been straight about just how close he was to senior executives at News International and it's time for him come clean about the extent of this relationship."
The question of whether Mr Cameron rode the horse became the subject of Westminster speculation after the loan by Scotland Yard to the ex-tabloid editor was made public.
After initially declining to directly answer questions, Downing Street aides conceded last night that the premier "probably" had mounted the ex-Metropolitan Police horse, shortly after Number 10 had to retract an earlier briefing that he had ridden with Mr Brooks after becoming Prime Minister.
His spokeswoman said that while he had "no recollection" of ever having ridden with Mrs Brooks, that could not be ruled out.
The case has refocused attention on the relations between politicians and newspaper executives, which are the present focus of the Leveson Inquiry into media ethics.
Mr Cameron has previously admitted that he became too close over recent years to executives at News International and other media organisations.
Asked whether the horse riding was emblematic of those overly close ties, Mr Cameron said: "I have known Charlie Brooks, the husband of Rebekah Brooks, for over 30 years.
"He is a good friend and he is a neighbour in the constituency. We live a few miles apart."
Details of the two-year loan of the horse were given last month to the Leveson Inquiry.
Mrs Brooks, who resigned last year as chief executive of News International amid the furore over phone-hacking allegations, "fostered" the horse after it retired from active service in 2008.
She paid food and vet bills until Raisa was rehoused with a police officer in 2010, months before fresh investigations began into illegal activities at the News of the World.
The force said the horse was returned in a "poor" condition and it later died of natural causes.
Mrs Brooks remains on bail after being questioned by detectives, days after resigning last summer, on suspicion of phone hacking and corruption.
Mr Cameron said in an interview yesterday that he had ridden once since becoming Prime Minister but Number 10 has declined to discuss the circumstances.
London Mayor Boris Johnson said he had not ridden the horse.
"I count myself proudly as a non-member of any kind of Chipping Norton set," he told BBC London, in a reference to the area of Oxfordshire where Mr Cameron and Mrs Brooks live.
But another well-known figure associated with that "set", Top Gear presenter Jeremy Clarkson, said he could "categorically state" that Mr Cameron had never ridden the horse either.
"It's all rubbish," he told BBC Radio 2 in an interview before Mr Cameron's confirmation.
Details of the horse loan were reported in the London Evening Standard earlier this week, leading Lord Justice Leveson to warn he may act to prevent leaks.
He warned that he could in future restrict advance release of witness statements to core participants: people who have a significant interest in the hearings or who may face criticism.