Grandparents should be given greater access to unpaid leave by their employers in order to look after grandchildren, the TUC says.
Nearly seven million grandparents provide regular childcare for their grandchildren aged under 16, the union says, mainly so the child's parents can work.
This informal care network saves working families thousands of pounds a year on costly nursery and childminding fees, as well as helping parents to stay in work and continue their careers, the TUC adds.
Working grandparents are more likely (63%) to look after their grandchildren than retired grandparents (55%), while but the TUC claims that employers of older people are failing to recognise this.
Around one in ten working grandparents have been refused time off work to care for grandchildren, while many more have felt unable to ask.
The TUC is demanding grandparents have greater entitlement to unpaid leave so they can combine looking after their grandchildren with their jobs.
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.
"The childcare provided by grandparents allows mums and dads to work, saves them money on nursery and childminder fees, and creates a special bond across different generations in a family.
"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."