Nearly half of audit inspections carried out by the Financial Reporting Council (FRC) in 2017/18 found problems in the way pension schemes valuations are reported.
In a new report, the watchdog said valuations are made complex by the existence of multiple pension arrangements and risk transactions, such as partial buy-outs and longevity swaps.
This has led to a "significant risk of material misstatement", with some inspections unable to determine if actuaries were part of the audit process and how obligations were evaluated.
The findings come after the collapse of Carillion put auditors under the spotlight, with MPs describing the firm's annual reports as "worthless" as a guide to its true financial health.
"Auditors need to understand the work of actuaries inputting to their work and pay attention to assets as well as liabilities," FCR audit and actuarial regulation executive director, Melanie Hind, said.
"The valuation of pension obligations is complex requiring significant judgements and assumptions which carry the risk of material misstatement and/or manipulation."
The FRC research focussed on the reporting quality of pension schemes' balance sheets and related disclosures in 51 of its audit inspections in the 2017/18 financial year.
It concludes that auditors can improve by assessing the sensitivity of valuations to changes in assumptions, clearly highlighting the work done by actuaries and how conclusions are reached.
They should also obtain sufficient evidence to support the valuations of different investments, and pay attention to the reasons for obligations and assets in multi-employer schemes.
However, ARC Pensions Law's Anne-Marie Winton said the real issue is that firms with defined benefit schemes are not obliged to disclose ongoing liabilities or the larger solvency or buy-out deficit.
"The deficit figure in the accounts could be the tip of the iceberg," she continued.
"While this is not a case of false representation under current accounting standards, key information about the risk to the company is not being included in its financial statements."