Ministers were warned about the dangers of private equity taking over the water industry in a briefing that has been kept secret for 20 years, according to the Guardian.

Details of the analysis are still being withheld as sewage pollution and the failure of water companies to invest in infrastructure are under national scrutiny.

On Thursday the water industry – after more than three decades running a privatised model – apologised for its failures to properly manage and invest in water, and for the scale of raw sewage discharges that have fuelled huge public anger.

It promised to triple funding in pipes, treatment works and infrastructure over the next decade to £10bn and apologised for polluting beaches and rivers with raw sewage. But all of this will be paid for by increased customer bills.

The report being withheld from publication predicted the state of the privatised water industry today, and warned against private equity being allowed to move into water firms.

It was prepared for the Competition Commission (now the Competition and Markets Authority, CMA) in 2002 and has never been published in full. It should have been released under the 20-year rule last summer, but despite repeated attempts to have it published it is being kept secret.

Today, as private equity dominates ownership of the water sector in England, bringing with it high levels of debt and underinvestment leading to sewage pollution, water shortages and leaks, the author of the report has called for full disclosure of his warning two decades ago.