Labour's shadow justice secretary Sadiq Khan pledged today to smash open the government vault stuffed with privateers' details.
He said that under a Labour government he would make contracts with the private sector subject to the Freedom of Information (FOI) Act.
But open government campaigners warned that he must back it up with action.
Currently the details of government deals with private firms are shielded from prying eyes, using clause that exempts information that would "prejudice commercial interests" or is consider a "trade secret."
That largely limits FOI requests to central and local government and state organisations such as the NHS or the police.
Private businesses and charities are not required to provide information at all - even where they are operating a public service with taxpayer funds.
Access to information on rail franchisees, private prison operators, Olympic organisers Locog and the secretive Association of Chief Police Officers - which until last year kept a controversial database on political dissidents - has frequently been a sore point with journalists and campaigners.
But the pledge carries even more weight with the rise of academies and free schools, as well as a £1.5 billion plan by West Midlands and Surrey police, who have since backed out, to outsource potentially all police support jobs.
Documents leaked in March revealed they wanted to flog off all services that "can legally be delegated to the private sector."
Mr Khan said today he was proud Labour had introduced the Act, "however awkward it can be."
He said: "For the first time, the Act will cover the delivery of public services by private companies. This includes our prisons, our schools and our health service.
"Public private or voluntary, subjected to the same disinfecting transparency," he said.
Open government campaigners appeared to greet the news with cautious optimism.
Spinwatch director David Miller told the Morning Star it was a relief "after all the talk that there's been over gutting the Act or introducing charges."
But even public officials' responses were increasingly sparse: Labour would have to support any amendments with new powers for the Information Commissioner and tougher enforcement, he said.
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