David Cameron has been told he must reveal further details about the refurbishment of his Downing Street flat under the Freedom of Information Act.
Information Commissioner Christopher Graham has ruled that the Prime Minister must disclose what work was done on the bathroom to the apartment at Number 11, where he lives with his wife Samantha and their three children.
It emerged earlier this year that the Camerons had spent the entire £30,000 budget available to prime ministers annually for upkeep and improvements to his Downing Street flat.
Officials stressed that the costs charged to the taxpayer did not cover furniture, fixtures and fittings, but things such as electrics, plumbing, structural alterations and decorating. Additional costs in the refit beyond the £30,000 publicly funded allowance were met by the family.
Labour MP Tom Watson has been asking the Cabinet Office for details of the changes to the bathroom and any instructions given to contractors by Mr and Mrs Cameron.
The Sunday Telegraph disclosed that, having rejected the request, the Cabinet Office has now been told by Mr Graham's office that he "does not accept" its decision.
"In particular the Commissioner considers it unlikely that planning for refurbishment works at Downing Street would take place without the involvement of the Cabinet Office as the relevant Government department," he said.
The Commissioner also raised concerns about delays in responding to Mr Watson's request, which was submitted in June last year.
Mr Watson, the shadow Cabinet Office minister, said: "Downing Street is a Grade One listed building and one of the jewels in the nation's crown. Taxpayers have a right to know what the temporary residents are doing to such a historic building."
A Number 10 spokesman said Mr Graham's letter had been received.