House of Commons speaker John Bercow has been accused of suppressing reports that reveal the scale of MPs' alcohol problems.
Documents which are thought to show concerning levels of drinking in Westminster's subsidised bars will not be published, after Bercow invoked a controversial loophole.
One MP claimed in 2011 that her colleagues drank "really quite heavily", and Commons bar staff have been given extra training on how to refuse to serve drunk customers, and serve less alcohol at events.
Alcohol Concern has urged parliament to remove subsidies that make booze cheap in the bars for MPs and peers.
There are around a dozen bars and restaurants on the estate serving politicians, staff and other passholders, and the taxpayer subsidy of some £4 million a year means a pint of beer costs as little as £2.90, far lower than normal London prices.
The House of Commons is believed to have looked into the issues and how to fix them, but Bercow has been accused of dodging scrutiny by claiming that the reports detailing the scale of the problems are exempt from any "public interest" test because they could be damaging to the workings of parliament.
The request follows a series of incidents that highlighted the issue of alcohol at parliament.
Former MP Eric Joyce was convicted of assaulting a fellow politician during a brawl in Strangers' Bar in 2012, while another ex-member, Mark Reckless, confessed to missing a late-night parliamentary vote in 2010 because he was too drunk.
Dr Sarah Wollaston, Totnes MP and now health select committee chairwoman, warned in 2011 that some of her colleagues were drinking "really quite heavily".
"Who would go to see a surgeon who had just drunk a bottle of wine at lunchtime? But we fully accept that MPs are perfectly capable of performing as MPs despite some of them drinking really quite heavily," she said.
After the death of former Liberal Democrat leader Charles Kennedy last year, his partner said his alcoholism was exacerbated "a lot" by Westminster's drinking culture.
The only information the House did disclose was that nine appointments were made with the Health and Wellbeing Service over "alcohol dependency" in September 2012, and none the following month.
The Speaker has invoked a loophole in the Freedom of Information (FOI) Act, to claim that information requested by The Press Association about drinking among MPs and peers must be kept secret.
The FOI request asked for "any evidence or reports produced by the Safety, Health and Wellbeing Service regarding the provision and consumption of alcohol on the parliamentary estate, and related health effects".
But the response from parliament's authorities was that Bercow believed releasing the information would "inhibit the free and frank provision of advice and the free and frank exchange of views for the purposes of deliberation."
The reports would also "prejudice the effective conduct of public affairs," his response claimed, meaning that they are exempt from the "public interest test" which usually compels public bodies to release information.
The House of Commons is the only public body that can use the Freedom of Information Act's Section 36 exemption, which Bercow has applied, without having to prove that it is in the public interest to keep information secret.
The Information Commissioner cannot even analyse the reasons for the Speaker's decision, as a certificate signed by him constitutes "conclusive proof" the exemption has been properly applied.
The same mechanism was deployed three years ago to avoid revealing details of Mr Bercow's tax bill for his grace-and-favour residence, which had been requested under FOI by the Press Association.
Maurice Frankel, director of the Campaign for Freedom of Information, said Mr Bercow appeared to be "avoiding scrutiny" to prevent damage to the reputation of MPs.
"He is exploiting a loophole in the FOI Act which parliament itself has inserted to protect parliament from scrutiny," he said.
"On the face of it there is no reason why they should not reveal what their assessment of any alcohol problem in parliament is.
"It is a matter of public interest if any MP's or peer's conduct is being impaired. It is entirely reasonable for us to know whether they regard that as a problem, and what has been considered.
"It is extremely easy for parliament to avoid scrutiny under the FOI Act."