More than 100,000 workers are not being automatically placed into a pension because they are juggling multiple jobs - and women are particularly likely to be missing out - according to Citizens Advice.
Around seven in 10 of those affected were found to be women, with female workers often having to find part-time jobs that fit around their family commitments.
Citizens Advice found that across the UK, nearly 106,000 working people are not being auto-enrolled because their earnings come from multiple jobs. It found that within this total, 72,000 women are affected.
The earnings trigger for a worker to be automatically put into a workplace pension by their employer is £10,000, but those highlighted by Citizens Advice are not being put into a pension because they earn at least this from several jobs combined, rather than from one single job.
Citizens Advice analysed figures from the Office for National Statistics' (ONS) Labour Force Survey to calculate the numbers of employees working multiple jobs.
Automatic enrolment started in 2012, to head off fears of a looming old age savings crisis. Employees taking part get the benefit of pension contributions from their employer.
So far, the scheme has generally been seen as a success, with around nine in 10 of those who are eligible for auto-enrolment choosing to stay in their pension rather than opting out.
By 2018, around 10 million people are expected to be newly saving into a pension or saving more as a result of the landmark initiative.
In December, the Government announced a review of automatic enrolment, which will explore ways in which the policy can be developed to further encourage workplace pension saving.
The review will look at the issue of people with multiple jobs who do not qualify for automatic enrolment in any single job. It will also consider how the growing numbers of self-employed people can be helped to save for their retirement.
Citizens Advice also commissioned a survey of more than 1,100 managers from businesses across England and Wales. The findings suggest businesses are more concerned about finding and keeping good members staff than they are about pension contributions being a burden on their business.
Some 18% of those surveyed said pension contributions are a key concern for the future, while 44% are most worried about finding good staff and 36% are worried about retaining them.
Gillian Guy, chief executive at Citizens Advice, said: "Too many people are shut out of a workplace pension - despite earning enough to qualify.
"Many people - particularly women - work several part-time jobs, which helps them manage commitments like childcare or study. 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.
"The Government needs to seize the opportunity of this year's auto-enrolment review and use it to pave the way for helping more people get on track with pension savings."
A Department for Work and Pensions (DWP) spokesman said: "Automatic enrolment has been a huge success with more than seven million people now saving for a workplace pension.
"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."