The warning comes after a landmark legal case involving a Spanish au pair whose host family were ordered to pay her almost €10,000 in back pay and compensation after a five-month stint in their home.

Under the ruling by the Workplace Relations Commission, an au pair has been established beyond doubt to be an employee like any other and therefore entitled to the minimum wage, holidays, rest periods, and other employment protections.

Forty similar cases are pending, including one involving a claim for €35,000 by an au pair who completed a two-year placement with an Irish family.

Some 20,000 households in Ireland are estimated to use au pairs as families try to find ways around the costs of formal childcare. Placement agencies have regularly advertised au pairs for €100 a week for 20-30 hours’ work plus board.

Now agencies are warning that the ruling will drive the arrangements further into the black economy.

Caroline Joyce of the Cara International Agency said: “Families will not pay the minimum wage because they can’t afford to.

“They won’t use reputable agencies who are ensuring standards and providing training in child protection. They’ll go online and make arrangements directly and standards will fall.”

Ms Joyce said it was unrealistic that the food and board provided was valued at only €54 per week and she said it was wrong that the cultural exchange aspect of the au pair tradition was not acknowledged.

Au pairs had long been considered to exist outside formal employment arrangements with pay being termed pocket money.

The counter argument that they were employees was untested until the case taken by the Spanish au pair, with the help of the Migrant Rights Centre of Ireland.

She said she had been left “exhausted, depressed, and weak” by her experience with an Irish family for whom she worked 30-60 hours per week for a set weekly payment of €100.

Her victory raises questions over the treatment of other informal childcare providers, including grandparents and babysitters.

Virginija Petrauskaite, legal officer with the Migrant Rights Centre of Ireland, said she believed breaches of au pair rights were commonplace. “There is a childcare crisis in this country, but exploitation is not the solution,” she said.