It is the most unlikely of generational revolts, but one that older Britons might just relate to. A call has gone out for grandparents around Spain to down tools this Wednesday and go on strike – because they're sick to the teeth of all that childcare.

For a country where half of all grandparents look after their grandchildren every day – one in eight for more than nine hours a day – the strike call threatens to disrupt the working lives of a significant proportion of the population and expose the extent to which this unpaid work is propping up the economy.

"We want grandparents to strike to prove they are a key part of the way this country functions," said Manuel Pastrana, leader of the UGT general workers' union in the southern region of Andalucia. "Learn to say no" and "don't feel guilty" are the slogans, aimed at so-called "babysitter grandparents".

The call is part of a wider attempt to bring the country to a halt with a general strike. But it has also struck at one of the key elements of Spanish society – where grandparents provide the childcare that working parents cannot give and the state does not offer.

"It is a growing problem because grandparents are cheaper than childminders and they are an easy option when the economy is as bad as it is now," said Dr Jaime Rodríguez, of Spain's Society of Gerontology. "That is probably fine for most of them, but some cannot cope."

One Madrid grandmother, Manuela Martin, 73, looks after seven-year-old granddaughter Antía all week, only giving her back to her parents for the weekend.

"She comes back to me on Sunday night and stays until Friday," she said. "Her parents start work at 7am so they can't get her to school during the week."

Spanish working hours are partly to blame. Long lunch breaks mean office workers have to work late. An office culture of not leaving before the boss goes home keeps some there even later. And with part-time or flexible jobs a rarity, many families with two working parents simply cannot function without a grandparent.

"Here they are full-time," says María Teresa López, a professor at the economics faculty of Madrid's Complutense University. "They are filling in for parents, effectively bringing their grandchildren up and that is quite a different role to being a grandparent."

It is not just Spain's working classes who depend on their relatives. "We are there when we are needed," says Juan José, 67, as he walks his three and five-year-old grandsons to a private nursery school in Madrid. "In the summer we have them at our home near the beach for almost the whole three months of the holidays while my son and his wife work. We love it, but our parents did not do this for us."

Doctors say being involved with grandchildren keeps most grandparents healthy and active, but those who do not enjoy the work will not admit it. Few are prepared to criticise either their own children, or their grandchildren. "I don't mind helping, but we have already brought up our own children and now we deserve some space of our own," said one grandmother, who did not want to be named.

But few said they would heed the call to strike. "What do they think we are going to do with [our grandchildren]?" asked Martin. "Do they want us to dump them in the street?"