Part-time workers with flexible schedules end up doing more work without pay.

The family-friendly hours where workers control their own schedules often end up working more after-hours the research from the University of Kent found.

On average in the UK men work an extra 2.2 hours a week in unpaid overtime. Women work for about 1.9 hours.

But Kent's Dr Heejung Chung and Dr Mariska van der Horst from the Vrije Universiteit Amsterdam, wanted to see if flexitime, teleworking or the ability to set their own hours encouraged workers to put in more unpaid overtime.

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And although flexitime and teleworking did not increase overtime hours, for those who could control their own schedules worked more: professional men about an hour and women without children about 40 minutes.

And although full-time working mothers did not seem to increase unpaid overtime hours, part-time working mothers worked an average of about 20 minutes more a week.

"Part-time working mothers [may] feel the need to work longer to compensate for the possible stigma, perceived or otherwise, attached to them by other workers, especially when their schedules do not match normal working hours," the researchers suggested.

"It may also be that part-time working mothers have a greater ability to work unpaid overtime compared to full-time working mothers."

Chung's previous research has found that two-fifths of part-time workers – more than half of whom are mothers – believe their reduced hours have had a negative impact on their career progression.

"More control over your work is supposed to make life easier for workers, particularly those with children," Chung says.

"However, it is clear that for many, blurring the boundaries between work and home life expands work to be longer, even when it is unpaid."

She said employers needed to make sure staff were not over-stretching and undoing the benefits of flexible working.

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She added: "We also do not find much evidence that those who take up flexible working arrangements reduce their workloads.

Employers need to be made more aware of this, to tackle our perception against those working flexibly."