As many as 1.75 million workers could be missing out on tax relief worth £60 million overall.
That's according to figures from Royal London , who submitted a freedom of information request to HM Revenue and Customs (HMRC).
It is thought that around three-quarters of these workers are women in low-paid or part-time jobs, and based on previous select committee evidence that each worker is missing out on an average of £35 a year.
Sir Steve Webb, Royal London director of policy, said: "It is a scandal that so many low-paid and part-time workers are missing out on tax relief on their pension contributions.
"This is the group that most needs a boost to their pension savings.
"These new figures suggest that the scale of the problem is much bigger than previously thought."
Brits about to start saving more by accident - and get a pay rise as a reward
The problem
When saving into a workplace pension, the money comes out of your pre-tax salary. That means £100 of pension contributions cost a basic-rate taxpayer £68 in real terms.
However, the threshold for enrolling people in a workplace pension is £10,000 - significantly less than the new tax threshold of £12,500.
That means someone making £12,000 a year automatically loses £528 from their take-home pay in pensions contributions.
But someone making £15,000 sees their take home pay drop by less (£510), but gets £240 more added to their pension fund as a result of tax relief.
And a lot of people fall into that gap.
HMRC told Royal London that 1.33 million people fell into this category in 2016-17, but since then there has been a large increase in the tax free personal allowance - dragging more people into the situation.
Royal London's projections now put the figure at an estimated 1.75 million for 2019-20.
The good news is, in some arrangements, workers might still be able to benefit from tax relief offered as standard to others.
If the pension scheme is delivered using a method called "relief at source", tax relief applies, but other schemes may exclude tax relief for those earning under the tax threshold.
The bad news is that the type of pension you're automatically enrolled in is something your boss decides, not you, and most workplace schemes don't use it.
