HM Treasury is failing to monitor "excessive" profits from the selling-on of PFI (private finance initiative) equity, the BBC has been told.
One industry analyst says its "inadequate" records do not reflect the billions of pounds made in the so-called secondary PFI market.
Critics, including some MPs, say the taxpayer should benefit from a share of these additional profits.
In a statement the Treasury said it had some information on most sales.
Criticism about the lack of information held by the government comes as two influential parliamentary committees prepare to take an in-depth look at PFI.
Today the Treasury Committee will hear evidence in its ongoing inquiry into the future of PFI, asking whether it has been value for money for the taxpayer. The following day, the Public Accounts Committee will look at the lessons learned so far from the roll-out of PFI.
More than 700 hospitals, schools, prisons and other public sector projects have been built under the PFI scheme.
But in many cases, the construction and investment companies involved have sold on their equity shares to infrastructure funds on the secondary PFI market.
HM Treasury keeps a database of such transactions, but according to analyst Dexter Whitfield, it is out-of-date and contains just a fraction of the information it should.
In a report produced by the European Services Strategy Unit think tank, Mr Whitfield said: "Government monitoring of the sale of equity in public private partnership companies is inadequate, infrequent and underestimates the scale of transactions.
"Meanwhile, banks and construction companies are ratcheting up large profits extracted from what is ultimately publicly-financed investment."
In a statement, the Treasury said: "The Treasury collects and updates data biannually from departments on changes of PFI share ownership, and this information is published on our website."
"We now have some form of equity holder information on around 81% of PFI projects."
In the case of the Calderdale Royal Infirmary in Yorkshire, Mr Whitfield said the Treasury database held no information at all on secondary equity sales.
But after scouring company accounts and other documents, he found shares in the hospital had changed hands nine times since 2002.
The BBC asked five of the companies which had sold equity in Calderdale Royal to disclose the profit they had made from the deals, but was told the information was commercially confidential.
'Wealth machine'
As part of a comprehensive piece of research into the market, Mr Whitfield discovered the average profit margin on PFI equity sales was more than 50%.
Analysing a sample of 154 projects, he found profits of more than £500m. If the same level of profit had been achieved by all PFI equity transactions, he estimates private sector profits would stand at £2.2bn.
Describing the profits as "excessive", he said: "It's a wealth machine. It's not necessarily printing money, but it's virtually that, given the scale of these profits."
His call for a clearer picture of the market was echoed by the chair of the influential Public Accounts Committee, Margaret Hodge, MP.
"There has to be transparency around the system, so that if there is some profit over time in the funding of these PFI contracts, that profit can be shared between the taxpayer and the private investor," she said.
Another Public Accounts Committee member, Ian Swales, MP, said the large profits made raised serious questions about whether the deals to finance, build and maintain hospitals and schools under PFI were good value for money for the taxpayer in the first place.
"By definition, that means the taxpayer got a bad deal at the start, or there wouldn't have been these super-profits to be made."
David Metter, chairman of the Public Private Partnership Forum, an industry body for the PFI industry, said the profits made were a fair reflection of the risks taken on by the financiers and builders of the projects.
"The private sector is efficient and has demonstrated in the past 15 years that it can deliver 700 projects on time and on cost," he said.
"It would be extremely surprising if the government decided to do it another way."
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