The Asda Mumdex survey of more than 1200 mothers found grandparents provide 24 hours of free childcare per week on average.
It found grandparents provided significantly more care for children than other common sources, with school and community groups providing eight hours of free childcare a week on average, other family members providing six hours and childminders four hours.
Neighbours and friends were also found to be providing two hours of free childcare a week on average.
Premier's Political Editor Martyn Eden is a grandfather. Commenting on the £6.5bn figure, he said: "Put that figure in the perspective of what it costs the taxpayer as result of supporting families that have broken down - 43 billion pounds a year.
"It's very important increasingly where people are paying such high mortgages that they need both partners to work, and so grandparents are there to fill in.
"A grandparent offers something that the parent doesn't. The parent is ultimately a source of discipline in the family - grandparents can spoil the grandsons and granddaughters a little bit. We can give them love without having to be there to do the discipline bit.
"It's historically the whole function of family. We've got too used to the family being dispersed and generationally broken up. It's very important that families stick together."
Listen to Premier's Political Editor Martyn Eden here:
