More than 1.6 million people have already been placed into pension schemes under the Government's automatic enrolment initiative, which is expected to recruit about 11 million new pension savers as it rolls out over the next five years.
But in her report, Dr Altmann said: "The future for pensions is more complex and risky than ever before."
She called for an overhaul of defined contribution (DC) pensions, which, she argued, "are not fit for 21st-century lives".
DC schemes are the type people are most likely to be placed into as the reforms roll out. They have largely replaced "gold-plated" pensions such as final-salary schemes where the employee was guaranteed a set income on retirement.
Workers paying into DC schemes take on more of the risk when it comes to the eventual size of their pension income.
The report warned: "The risk of buying at the wrong time, choosing the wrong annuity or failing to find the right rate could increase the number of poorer pensioners by many millions."
The Office of Fair Trading (OFT) recently advised the Government to look at improving the transparency and comparability of different pension schemes, to make it easier for employers to make the right choice.
The OFT found up to £40 billion of pension savings could already be in schemes that are delivering poor value or are at risk of doing so.