Friday's High Court ruling is yet another setback for MPs in their efforts to fend off scrutiny of their expenses.
An unintended consequence of the Freedom of Information Act introduced by Tony Blair in 2000 has been the emergence of ever greater detail about how MPs spend their allowances.
MPs last year claimed a total of almost £90 million - around £135,000 each - for expenses ranging from office staff to IT, stationery, travel and stamps.
But Friday's ruling relates to perhaps the most sensitive aspect of the system - the Additional Costs Allowance, which is worth up to £22,110 a year.
Commons rules state that ACA is paid "to reimburse Members for necessary costs incurred when staying overnight away from their main home for the purpose of performing parliamentary duties".
Most MPs use it to pay for the cost of running a base near to Westminster while away from their constituency homes, and it cannot be claimed by those representing seats in central London.
There have been complaints that ministers who enjoy the use of grace-and-favour accommodation - like the Prime Minister's flat at 10 Downing Street - are nonetheless permitted to claim ACA to pay for the mortgage on their constituency homes.
And there was controversy earlier this year with the revelation of the so-called "John Lewis list" kept by Commons authorities as a guide to the level of claims MPs can make for furnishing and renovating their second homes, based on the prices at the famous London department store.
These included items such as £10,000 for a new kitchen, £6,000 for a bathroom, £750 for a TV or £300 for rugs.
Since 2001, the total ACA claimed by each MP has been published as part of the annual expenditure allowance table, but journalists and freedom of information campaigners have used the FOI Act to try to find out exactly what the money is spent on.
After a series of rulings by Information Commissioner Richard Thomas, the Commons authorities accepted earlier this year that a breakdown of the ACA claims would have to be released.
A breakdown of the claims from a handful of individual MPs named in FOI requests was released in April, revealing for example that Mr Blair claimed £116 for his TV licence, Gordon Brown £2,000 for cleaning and ex-Deputy Prime Minister John Prescott £4,000 for food in the space of one year.
Details of all MPs' claims for the years 2004/05 to 2007/08, broken down by category, are to be made public in the autumn, and in future the information will be released on a quarterly basis.
However, the Commons Members Estimate Committee, chaired by Speaker Michael Martin, continues to object to the Information Tribunal's ruling that all documentation relating to claims, including invoices and receipts, should also be made public.
The committee argues that this would inevitably mean the addresses of MPs' second homes being made public, unfairly breaching their right to privacy.
Now that this argument has been rejected by the High Court, the Commons must decide whether to take its fight to the Court of Appeal or allow the spotlight of publicity to be shone ever more brightly into MPs' private lives.