Commons Speaker John Bercow has been criticised for suppressing details of discussions about the tax bill for his grace-and-favour residence.
Mr Bercow has taken the unusual step of issuing a certificate that effectively exempts material from normal Freedom of Information (FoI) rules.
The move came after a request by the Press Association regarding the tax treatment of a handful of prestigious residences granted to senior figures including the Speaker, Clerk of the House and Serjeant-at-Arms.
The prime Westminster properties are worth millions of pounds. Living accommodation provided to staff by employers is usually treated as extra income by the taxman.
HM Revenue & Customs considers whether the accommodation is necessary for the job. The taxable benefit can reflect the full rental value, or merely running costs such as utilities - as has happened with the Prime Minister and Downing Street.
Commons documents show that Mr Bercow incurred a taxable benefit of £4,300 for Speaker's House in 2009-10, with the figure worked out using a "formula based on a percentage of annual taxable salary and period of residence".
The same year the Clerk of the House's use of 3 Parliament Street was worth £27,000 to him, according to the annual Commons accounts.
However, talks with HMRC resulted in the assessment of the benefit for both Mr Bercow and the Clerk plummeting to nothing in 2010-11. The accounts stated that "it has been agreed that no benefit in kind liability arose".
The latest accounts, for 2011-12, again list no taxable benefit for the Clerk, but add that the issue "is currently being discussed with HMRC". The change suggests that the tax bill for the Speaker and his senior officials may have been significantly reduced - although the situation is not clear.
Matthew Sinclair, Chief Executive of the TaxPayers' Alliance said: "It's completely inappropriate for the Speaker to make the final judgement over whether legitimate questions about his own taxpayer-funded perks are of public interest or not. His use of this exemption appears to be a blatant attempt to shut down a perfectly reasonable investigation into how taxpayers' resources are being used."