It said that a staggering seven million grandparents currently provide free childcare to grandchildren aged 16 and under - the equivalent of 63 per cent - with around one in five grandmothers providing at least 10 hours of care a week.

Half of all mothers also rely on grandparents to look after their babies when they return from maternity leave.

But with the state pension age set to rise and increasing numbers of grandmothers expected to stay in work until they are 65 or older, many will be unable to give their own children the support they need.

Grandparents Plus interim chief executive Denise Murphy warned: "With childcare costs soaring, many parents are becoming increasingly dependent on grandparents to look after children so they can go to work.

"But we think one of the consequences of the raising of the state pension age may be more mothers giving up work because grandmothers are no longer available to provide childcare."

The study, which used data from the 1998 and 2009 British Social Attitudes Survey, found the highest percentage of childcare is provided by grandmothers aged 55 to 64, followed by grandparents aged 65 to 74.

Around 60 per cent of grandmothers who look after their grandchildren are aged under 65.

Ms Murphy added: "As the UK population ages there is growing pressure on grandparents to go out to work, as well as provide care for their grandchildren and often, elderly relatives as well.

"As older people remain longer in the workplace, there is a risk of a serious 'care gap' emerging in the provision of informal care for children and older people."

The charity is calling on grandparents who care for their grandchildren to have access to flexible working