Many larger firms offer to "match" additional pension saving by their employees, by adding a contribution when workers save more.
But insurer Royal London said that many workers were unaware of the option, and failed to take advantage.
This could make a difference of hundreds of pounds in retirement income, it said.
Despite having to make a number of assumptions in calculations, Royal London has estimated that an estimated £2bn of employer pension contributions would be available if workers took up the option of saving to a maximum rather than minimum level.
It estimated that 3.2 million workers could receive an extra £650 per year each.
That would mean somebody on average earnings who chose to take up an additional 3% matched contribution to their pension pot would receive a retirement income of £22,500 rather than £19,050, thanks to the extra contribution and tax relief.
The Nationwide Building Society has changed its default level to a maximum, rather than a minimum contribution. Steve Webb, a former pensions minister and now director of policy and Royal London, said more could be done to make workers at other businesses aware that their employer might add to their pension if they were willing to contribute at a higher level.
"At a time when money is tight for many people and pay rises may be limited, getting your employer to contribute more to your pension can be a very cost-effective strategy," he said.
"When individuals are thinking about where to put their money to get the best return, the chance to more than double your money through an employer contribution and tax relief from the government takes a lot of beating."
Graham Vidler, director of external affairs at the Pensions and Lifetime Savings Association, said; "If your employer offers a workplace pension, you should certainly consider joining and putting in as much as possible as if you don't you will miss out on free money in the form of employer contributions as well as the tax relief.
"This research highlights how much UK employees could be missing out on each year. It is also vitally important that people start saving into a pension as soon as possible as the longer you save, the bigger your final pot is likely to be and the more you can benefit from your employers 'matching' contributions."
Workers are missing out on pension top-ups from their employers by only saving at a minimum level, research suggests.