Tens of thousands of women are missing out on workplace pensions as a result of having more than one job, according to Citizens Advice.
To qualify for an auto-enrolment pension, workers have to earn at least £10,000 a year.
But more than 100,000 people - most of them women - do not reach that threshold, because they work for several employers.
The government said that it is planning to review the issue later this year.
A separate study shows that women still receive far smaller pensions than men.
According to the insurance company Zurich, the average woman will have £47,000 less in her pension pot than a man by the time she retires.
Citizens Advice said that 72,000 women were missing out on auto-enrolment pensions, which require employers to pay a pension automatically, unless a worker deliberately opts out.
The charity said too many people were being shut out of the opportunity to be paid a pension.
"Many people - particularly women - work several part time jobs, which helps them manage commitments like childcare or study," said Gillian Guy, the chief executive of Citizens Advice.
"But while in many cases they earn over £10,000, and pay tax on this combined income, they don't have access to a workplace pension and miss out on the opportunity to save for their retirement."
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The government said in December that it would examine the issue of workers with multiple jobs when it reviews the auto-enrolment programme later this year.
"There's more to do - especially for people with more than one job - and we're currently reviewing the policy to see how it can be improved," a spokesperson for the Department of Work and Pensions said.
The Zurich analysis found that between 2013 and 2016 men received 7.8% of their salary in pension contributions on average, compared to women receiving 7%.
It said men tend to work in sectors with more established or generous pension schemes.
In addition, women are more likely to take career breaks.
"This difference in the contributions that they receive from their employer presents a serious - and growing - problem," said Rose St Louis, Zurich's head of partnership development.
"The triple effect of smaller salaries, career breaks for women and lower contribution rates needs to be addressed: we can't ignore a £47,000 shortfall," she said.