As much as £25.8 billion-worth of pension savers' money is at risk of being exposed to high charges, an independent body has warned.
The Independent Project Board (IPB) made the findings after overseeing an inquiry into older and higher-charging pension schemes which examined £67.5 billion worth of savers' assets.
It found that between £23.2 billion and £25.8 billion of assets are potentially exposed to fees which equate to an annual management charge of above 1%.
Pensions Minister Steve Webb, who said he will be asking providers to take bold action in response to the findings, said: "I think this report shows what I'd call the guilty secrets of the pensions industry writ large and the onus is now on the industry to put its house in order."
Mr Webb also described it as "shocking" that people with smaller pension pots seem to be at risk of the highest charges.
Between £5.6 billion and £8 billion-worth of assets are potentially exposed to charges above 2% and around £900 million are potentially exposed to charges above 3%, the report found. Schemes where savers are potentially exposed to the very highest charges are more likely to have complex fee structures.
Of the £900 million worth of assets which is exposed to charges over 3%, most is held by savers with pension pots of less than £10,000. The report said that for such savers, the impact of monthly deductions can be "very high".
Mr Webb highlighted the fact that the report had found 38 different types of charges and and 291 different combinations of charges.
He said that although the Government is set to introduce a charge cap that will help savers in the future, "we clearly need to do more for the money that's already in high-cost pensions".
He said: "I'm going to be contacting all the big providers, getting them in, asking them for big, bold responses to this report. We can't spend two years sorting this out and I think the onus is now on the industry to get its act together."
Thousands of savers in schemes with the potential for high charges had only joined them relatively recently.
The IPB estimates that 407,000 savers who have joined schemes in the last three years could be exposed to a charge of more than 1% in the future. Of these, 178,000 could be exposed to charges over 2% and 22,000 are at risk of charges of over 3%.
Meanwhile, around £3.4 billion worth of pension assets have potential exit charges of 10% if savers left their scheme now, the report found.
Of this, around £800 million is held by savers aged over 55, who will be eligible to take up the new freedoms which will be offered from April 2015 to people to take their pension pot how they wish to, without being herded towards buying an annuity.
The remaining £2.6 billion is held by savers who cannot withdraw their pension savings but may wish to transfer them to a better value scheme, the report said.
The audit covered defined contribution (DC) workplace pension schemes set up before 2001 and administered by members of the Association of British Insurers (ABI), as well as more recent schemes with fees equating to more than a 1% annual management charge and schemes with multiple fee structures.
It follows fears raised by the now defunct Office of Fair Trading (OFT) last year that some savers' cash may not be achieving value for money. As a remedy, the ABI and its members agreed to undertake an audit of schemes and report to an independent board.
The IPB concluded that there is no "one size fits all" charge structure that will ensure savers always get value for money.
But it has written to providers with schemes where savers are potentially exposed to high charges or exit fees.
By June 30, providers should have identified what can be done to improve outcomes for savers and to stop new savers joining poor value schemes. Independent governance bodies will also need to agree a plan of action with their provider by the end of 2015 at the latest.
The IPB has also recommended that the Department for Work and Pensions and the Financial Conduct Authority should jointly review progress into remedying poor value schemes and produce a report by the end of 2016.
Tom McPhail, head of pensions research at financial services firm Hargreaves Lansdown, said: "Whilst the audit committee didn't have the power to force the pension providers to put their house in order, it is clear that if they don't do it of their own free will today, they may be forced to tomorrow.
"In the meantime, any investor who thinks they may have pension money in one of these old contracts should look at whether they would benefit from moving it elsewhere. Check first whether you'll pay a charge to move it."
The IPB includes representatives from regulators and the world of finance. Its chairwoman is Carol Sergeant, a former chief risk officer at Lloyds Banking Group, who also led a Government review into simple financial products.
Huw Evans, the ABI's director of policy and deputy director general, said: "Providers will welcome the clarity this report provides and will remain absolutely committed to building on the radical changes of the last decade which have already seen average pension charges fall to their lowest-ever levels for auto-enrolment schemes.
"This report will help providers do more to identify and tackle those workers who could be impacted by higher charges and ensure the right outcomes for them."
The Government plans to impose a 0.75% cap on management fees next April. This is to ensure that as people are automatically enrolled into a workplace pension, they can have confidence they will be placed into a scheme offering good value and which will not eat big chunks out of their savings through high charges.
The pension savers who were the focus of the IPB's report are those who have money in schemes which are not used for auto enrolment and savers who will have stopped contributing before April 2015 or the date that the scheme is used for automatic enrolment, if later.
Read more on AOL Money
£47 billion pension crisis could hit council tax
Don't get caught in these pension tax traps
Pension firms misleading customers, says watchdog