Commons Speaker Michael Martin has vetoed a request for the names and salaries of MPs' staff who are paid for by the taxpayer, it has emerged.
Mr Martin issued the certificate banning disclosure in May 2006.
He says releasing such details would breach freedom of information laws because it was likely to "prejudice the effective conduct of public affairs".
Information watchdog Richard Thomas has ruled that there are no legitimate grounds for exempting the names.
However, he is powerless to act because Mr Martin has banned the disclosure.
Commons and Lords 'exempt'
Heather Brooke, director of the Your Right to Know campaign for transparent and accountable government made the original request to the House of Commons in January, 2005.
But it was almost a year and a half later before Mr Martin issued a certificate banning the disclosure of the names of MPs' staff, saying it would breach the Freedom of Information Act.
The commissioner can neither order the details to be released or make an assessment of whether doing so would be in the public interest.
At first the Commons refused the request by claiming it did not hold the information, but the commissioner found that it did.
The Commons also claimed that publishing the names of staff would be an invasion of staff privacy and would be a breach of their health and safety.
Mr Thomas ruled against the use of these exemptions, saying publication of the names was not unfair under the Data Protection Act, adding that many of the names were already in the public domain.
He also said the Commons had not submitted "any compelling or legitimate grounds for exempting the names of MPs' staff for health and safety reasons".
Ms Brooke says: "If the information was really prejudicial then why wait until the last minute to issue this certificate?
"For Parliament to exempt itself from the requirements of open government shows a real failing in our Freedom of Information Act.
"MPs are elected to serve the public and as such they should be at the vanguard of the public's right to know.
"Instead they have shown a love of secrecy that surpasses even the European Parliament."
The refusal follows the information commissioner's call for the Commons authorities to reveal the number of rail warrants issued to David Blunkett.
He also called for the release of a breakdown of travel claims made by Labour MP Anne Moffat and her husband.
The Commons have so far refused to release details saying it would contravene the Data Protection Act.
Their objections were dismissed by Mr Thomas following an appeal.