A Labour government would massively extend the scope of freedom of information (FoI) laws to cover private companies contracted to do public sector work.
Shadow justice secretary Sadiq Khan said FoI legislation would apply to private firms running prisons and delivering education and health services.
They would all be subjected to the "disinfecting transparency" of the FoI regime, Mr Khan said in his speech to the Labour Party conference in Manchester.
He said the exemption for private contractors was a "blind spot" in FoI laws, and added: "I'm proud Labour introduced FoI, however awkward it can be.
"Not only will the next Labour government protect FoI, but we will seek to extend it. For the first time, FoI 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 of FoI."
Mr Khan also promised to tackle the "morally wrong" situation of too many people with mental health problems being held in prisons.
"Over 70% of prisoners have two or more mental health problems," he said. "We've replaced the Victorian asylum with the Victorian prison.
"Festering in prison with serious mental health problems that can and should be treated is morally wrong.
"That's why, in the next Labour government, I want to open a new front in the war on re-offending.
"For the first time ever, we will give a justice minister specific responsibility for rooting out mental health problems in our criminal justice system."
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Mr Khan said Labour would also look at introducing a women's justice board to work with females in the prison and probation system.
He also said that under Labour, courts would have to consider the option of restorative justice as part of any sentence handed down.
Mr Khan accused David Cameron and Justice Secretary Chris Grayling of a shift to the right in order to placate restive Tory backbenchers.
"David Cameron is doing what a Tory leader does when in trouble: lurching to the right, tough rhetoric and, dare I say it, acting all butch.
"We can guess what the new Justice Secretary Chris Grayling will say to the Tory conference next week. The same old tired Tory crime and justice speech.
"All rhetoric and no action, no evidence of what actually works.
"Re-toxifying the brand, tossing red meat to Tory backbenchers, now in control of this weak Prime Minister."
