While one in five (20%) people aged over 55 who were still working did not know the value of their combined pension saving, this increased to 30% among those who had retired, research for Fidelity Worldwide Investment found.
The study found evidence to suggest many retired people could be storing up future hardship.
About 13% of over-55s did not know how they were going to fund essential outgoings, and three-quarters (76%) of people who said this were fully retired.
The findings were made from a survey of more than 1,000 people, of whom roughly two-thirds were retired and a third were still working, either full or part-time. Those surveyed had some form of retirement savings on top of a state pension, although their money was not necessarily held in a pension scheme.
Over-55s in the Midlands were found to be the most out of touch with their retirement savings, with more than one in three (35%) saying they did not know how much their combined savings were worth.
Nearly a third (31%) in Scotland and a similar proportion (30%) in Wales did not know the overall value of their pension savings.
More than half (55%) of over-55s in Scotland said they had not drawn up a budget for retirement and did not think one was needed.
People in London were the most engaged with their pension savings, with almost four-fifths (78%) knowing the value of their pots.
The findings come as many people take up new retirement freedoms launched in April to give people aged 55 and over more flexibility over how they access their pension cash.
The Government has set up the free service Pension Wise to help people decide how to use the new freedoms.
Richard Parkin, head of retirement at Fidelity Worldwide Investment, said financial apathy may increase when people retire as they simply feel it is too late to change their situation.
He said: "To a certain extent, they are not entirely wrong - they do have fewer levers to pull - but that doesn't mean that actions taken post-retirement won't make a difference.
"For anyone who may recognise themselves in this situation, you most certainly aren't alone, however I strongly urge you to use the next wet weekend to dig out everything on your pension savings and just check that you are spending what you think you are and can afford. It's not the most thrilling task but it is one of the most important. And it's certainly not too late."
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