Labour MP Tom Watson has written to David Cameron, saying he fears a "culture of dishonesty developing in Downing Street" in the wake of the revelations that the prime minister did indeed ride the horse loaned to Rebekah Brooks by the Metropolitan Police.
Raisa, a former police horse, was retired from service and in 2008 and was "loaned" to Rebekah Brooks, who lives in rural Oxfordshire. Reports suggested she returned the horse to the Met in 2010 "in a poor condition."
Confusing messages from Downing Street last week first suggested on Thursday that the PM had denied riding the horse.
Number 10 then rowed back from the denial, saying that Cameron had "probably" ridden Raisa before Rebekah Brooks sent the animal back to the Metropolitan Police because she no longer wanted her.
Finally Cameron himself confirmed that he had ridden Raisa, saying that a "confusing picture" had emerged about the sequence of events surrounding the horse, which has since died.
In his letter to Number 10 on Monday Tom Watson asks:
The Mail on Sunday claim they put a series of suggestions regarding horse riding in the company of Mr Brooks that were denied by Mr Andy Coulson. Did Mr Coulson discuss the matter with you before issuing the denial?
"At what times last week did Downing Street staff raise the concerns of journalists with you?
"What steps are you taking to establish whether the reports are true? You are on record as saying that, whilst working for you, Mr Coulson did "an excellent job in a proper, upright way at all times." If the investigation finds this claim is inaccurate, what steps are you taking to ensure that other potential incidents of wrongdoing are investigated?
"If these reports are true it would be a sign of, at worst, a culture of dishonesty developing in Downing Street or, at best, a failure by yourself to ensure the highest standards of integrity are maintained by your staff. In either case, how do you propose to address this?"
Number 10 has not publicly responded to Watson's questioning so far.