Grandparents should be entitled to take unpaid leave from work because of the growing numbers looking after their grandchildren, a new report has urged.
Research for the TUC showed that almost three out of five grandparents provided regular childcare, mainly so that the child's parents could work without having to pay costly nursery fees.
With record numbers of people now working into their late 60s, many are now taking on childcare responsibilities for a second time in their lives, said the report.
Working grandparents are more likely to look after their grandchildren than those who have retired, the study found.
Some grandparents said they had been refused time off by their employer or did not feel they could ask.
The survey of 4,000 grandparents and a similar number of parents found that the level of unpaid, informal childcare was saving families thousands of pounds a year, said the TUC.
General Secretary Frances O'Grady said: "The informal childcare that millions of grandparents regularly provide is one of the most important and unheralded forms of care in Britain today.
"But with more people than ever before working into their late 60s, millions of grandparents are selflessly taking on childcare responsibilities for a second time while they still work. Many businesses have yet to keep up with this trend and thousands of grandparents who want to look after their grandkids are prevented from doing so.
"It's important that public policy catches up with the needs of working grandparents and their families. A new right to unpaid leave would be a great way to get more working grandparents involved in childcare, and at very little cost to an employer."
Chief executive of Grandparents Plus Sam Smethers added: "Family life is changing and it's time that government and employers caught up.
"Grandparents are picking up the strain that families are under and providing an increasing amount of childcare. But they are under pressure themselves, working longer and struggling to combine paid work with caring.
"We risk a 'childcare gap' emerging, with parents paying the price, if grandparents cannot afford to reduce their hours or can't get the flexibility they need. The solution is a period of grandparental leave and an investment in formal childcare."
A Department of Business spokesman said: " The Government recognises the valuable role that grandparents play in supporting parents with childcare. This is why we're extending the right to request flexible working to all employees from April 2014.
"Making flexible working available to all will not just benefit grandparents but allow the wider family to help out with child-caring responsibilities. It will give all employees the opportunity to better balance work with their personal life."