There is insufficient holiday childcare in two-thirds of local authorities in England, Wales and Scotland, research suggests.
The Daycare Trust survey also indicates most councils cut their holiday childcare budgets last year.
Shortages are worst in rural areas, where parents were "almost entirely reliant on relatives" for childcare, according to the survey.
The average cost of a week's holiday childcare is nearly £100, it says.
But in some areas, such as the South East, weekly costs rise to £110.
The Daycare Trust report says the importance of accessible and affordable holiday childcare cannot be overstated.
"It is a vital service that allows parents to work and contribute to the economy through tax revenues," it says.
Under the 2006 Childcare Act, local authorities have a duty to ensure sufficient childcare in their area.
Daycare Trust chief executive Anand Shukla said this year's survey illustrated the "lottery" parents faced not only finding, but paying for childcare.
"Council cuts to holiday childcare budgets and Family Information Services are hitting families across Britain hard, with only one in three local authorities now providing the childcare working families need this summer, despite their legal duty to cater for the needs of all," she said.
"As a result, more parents than ever will face a juggling act to ensure their children are looked after this summer."
The Daycare Trust surveyed all councils in England, Wales and Scotland, and followed up with freedom of information requests. About two-thirds of local authorities, or 134, responded.
The report was published as the Department for Education invited people to suggest ways to improve the affordability and accessibility to quality childcare.
Sarah Teather, Minister for Children and Families, said: "I know at this time of year that parents across the country are concerned about holiday childcare cover.
"They want an affordable and high quality service. I also want to hear ideas on how we can improve the affordability and quality of childcare in this country."
She added: "Early education can help children advance and catch up, and help them prepare for school. It's one of the most important and powerful factors in determining a child's future."