This follows HMRC's confirmation yesterday (27 November) that it "knows that scheme members are forgetting to declare the details of their annual allowance charge" on self-assessment returns.
Taxpayers are asked whether they have breached the annual allowance limit when completing tax returns - currently £40,000 for most - but often do not factor in including growth on defined benefit pension rights as well as cash placed into defined contribution pots.
Royal London said there "has long been suspicion" that individuals who do not understand the system leave a black answer to the question.
Individuals' marginal tax rate - on which pension input above the annual allowance is then charged at - could be as high as 40% or 45%.
Policy director Steve Webb said: "The shocking saga around the annual allowance for pension tax relief gets worse; filling in this tax return question requires individuals to understand the system, especially if they are affected by the tapered annual allowance.
"Thousands of people could be set to face huge tax bills now because they have innocently failed to declare this information on their tax return."
HMRC in turn pointed to pension schemes to remind members of their duty to declare the correct data on their tax returns.
However, pension schemes will only notify members who breach the £40,000 cap, meaning those in the tapered annual allowance gap (between £10,000 and £40,000) could be left in the dark.
Webb said HMRC needed to "get to the bottom of how many people have failed to declare the information" by contacting them immediately if it is aware.
Professional Pensions is holding a live Q+A with UK Pensions Awards judges tomorrow morning to help you prepare your submissions.
In this week's Pensions Buzz we want to know if you think it should be the responsibility of the NHS to take emergency action on tapered allowance tax issues.
In our 2019 Real Asset Study, we spoke to 500 investment decision-makers across Europe, representing over €600 billion in assets under management. One undeniable trend highlighted in the study is the growing influence of environmental, social and governance...
Registration for the UK Pensions Awards 2020 is now open…