David Cameron confirms rode former police horse Raisa while it was on loan to Rebekah Brooks - but not while prime minister

David Cameron's efforts to edge away from Rebekah Brooks, the fallen News International executive, came unstuck on Friday when he was forced after three days of non-denial denials to admit that he had ridden on a horse loaned to her by the Metropolitan police as a personal favour.

He said he had not ridden the horse named Raisa while prime minister.

Disputes over the significance of the story rapidly became as much a matter of controversy as the story itself, and unleashed a crowded field of horse metaphors.

Cameron, who cultivates an image of middle class normality, will be horrified at the way the episode links him to a lethal cocktail of urban journalistic cynicism, patrician country pursuits, police corruption and Downing Street evasion.

Labour's deputy leader Harriet Harman seized on the imagery to claim: "People will be dismayed that while News International was busy hacking phones, David Cameron was out hacking with Rebekah Brooks's husband.

"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."

For three days the prime minister's spokesman had displayed ignorance, boredom and displeasure at requests to know whether Cameron had ever ridden Raisa, the former Met police horse loaned to Brooks between 2008 and 2010.

Brooks, who has been arrested by the police in connection with the phone-hacking scandal, was also reluctant to talk about any favours she had bestowed on Cameron.

The No 10 facade over horsegate had started to crack late on Thursday night. The previous line had been firm: "He has no recollection of ever going riding with Rebekah Brooks."

But by late on Thursday night, Cameron's memory was refocusing. One of the prime minister's aides said: "It is highly possible that he was on that horse. It is likely that he rode that horse."

On Friday morning, as the crisis in Syria descended into hell and European leaders met again to save their currency, Cameron took time off from EU summit in Brussels to refresh his memory even more.

Cameron said: "Let me shed some light on it. I have known Charlie Brooks, the husband of Rebekah Brooks for over 30 years, and he is a good friend. He is a neighbour in the constituency, we live a few miles apart. I have not been riding with him since the election.

"Before the election I did go riding with him. He has a number of different horses and yes one of them was this former police horse Raisa, which I did ride."

Cameron, who has a history of avoiding questions on when he met Rebekah Brooks – admitted he had flip-flopped on the issue.

He said: "If a confusing picture has emerged over the last few days, I am very sorry about that. I think my staff have had to answer a lot of questions about horses."

Cameron added that he was sad to learn that Raisa, who was given back to Scotland Yard in 2010, had since died. He said: "I am very sorry to hear that Raisa is no longer with us and I think I should conclude that I won't be getting back into the saddle any time soon." Number 10 later said he rode the horse more than once and knew it was a retired police horse.

But by now the story itself was now as episodic, unpredictable and as gripping as a 20-episode box set of The Killing. On the Met police website it stipulated that retired horses should not be ridden. "The Mounted Branch is looking for suitable homes for retired horses, that is homes where the horse will not be ridden", the Met advice read.

This placed Cameron under threat of being in breach of the Met's regulations.

Swiftly, the Met, obliging to the political class as ever, rewrote the regulations on the website, suggesting the retired horses, and their fitness to be ridden should be looked at on a case by case basis.

Further distressing detail emerged on Raisa. She had been traumatised by 13 years in the riot squad and too difficult for a novice like Rebekah Brooks to ride, suggesting Cameron is a good rider. She had been returned to the care of Scotland Yard in early 2010, before the general election, and sent to Norfolk where she died. It has been alleged that the horse had been badly treated, prompting the story to take another unexpected direction, with the late emergence in the plot of a new character in the shape of Jeremy Clarkson.

The friend of motorists and equines declared: "I saw that horse and it wasn't badly treated as some people were saying. It was beautifully treated, it was only there for a very short time".

He added that he himself had never ridden it but Rebekah Brooks "probably did and her husband did".