Parents are paying on average £100 a week for a nanny or £72 for a nursery place for their children, according to Government research.
High costs and an apparent lack of local childcare places mean m any families cannot afford to leave children with carers, the survey of 8,000 parents in England found.
Despite Government claims to have increased the stock of registered childcare places by 90 per cent, four out of ten parents reported shortages locally.
Half of families relied on grandparents to provide childcare - often for free, the study found.
More parents were turning to formal childcare than in previous years.
But affordability was an issue particularly in London, says the report compiled for the Department for Education by the National Centre for Social Research.
The study says: "Cost of childcare and early years provision was a barrier to its use for some parents, particularly low earning families and lone parents.
"Significant minorities of parents also cited as barriers an inadequate number of formal childcare places in the local area and insufficient services at weekends, evenings and in the school holidays."
The survey found:
* The highest weekly costs were for nannies or au pairs, at an average of £100 a week
* Day nurseries were the second most expensive option at £72 a week
* Childminders cost parents on average £36 a week, while nursery schools cost £22.25.
Families were much less likely to pay grandparents but when they did the costs were similar to nursery places, on average £20 a week.
Ministers have set out a ten-year childcare strategy which promises to provide "affordable, flexible, high quality childcare for all parents who need it".
The research suggests the Government's strategy "may well have helped to lift some families' incomes", enabling them to pay for childcare and find their own paid work.
A DfES spokesman said: "Since 1997 we have invested over £17 billion in early years and childcare and the number of regis-tered childcare places has almost doubled to 1.2 million. Nearly all three and four-year-olds now take up at least some of their entitlement to 33 weeks of free early learning and care."