Half a million mothers of young children may be "missing" from the UK workforce because of unaffordable and inflexible childcare, a report will reveal this week.

Maternal employment rates for British women whose youngest child is aged between three and five are below the average for the industrialised world, the study by the IPPR think tank says. Despite families being given 15 hours of free childcare a week for three- and four-year-olds, the cost of the rest of the time is putting mothers off returning to paid work.

Analysis of the latest figures for OECD countries by IPPR shows that the average employment rate for mothers whose youngest child is aged between three and five is 64 per cent across the developed world. The rate in Britain is six points lower, at 58 per cent, the equivalent of around 150,000 mothers not employed; in Sweden the figure is 80 per cent. If the UK's maternal employment rate matched that of women without dependent children, an extra 570,000 mothers could be in paid work.

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A part-time nursery place costs more than £100 a week, a rise of 77 per cent over the past decade, twice as fast as general prices. Since 2008, the number of child centres offering care for the whole day has halved. The majority of earning mothers with young children are in part-time jobs – 60 per cent work less than 30 hours a week, more evidence of what the IPPR has previously identified as the "motherhood pay penalty", where mothers in part-time jobs earn 22 per cent less per hour than women in full-time posts.

The coalition has pledged to extend the availability of 15 hours of free childcare, now open to all three- and four-year-olds as well as to the poorest 20 per cent of two-year-olds, to 40 per cent of two-year-olds. Labour has pledged to increase the free provision to 25 hours.

Dalia Ben-Galim, associate director at IPPR, said: "The Government needs to focus on supporting mothers into work as a priority and then make it easier for them to increase their hours. The current 15 hours of free childcare has been hugely beneficial to mothers already in work, but has been largely ineffective in supporting mothers back to work."