A local childcare charity has said it is concerned about the use of unregistered childminders.
Northern Ireland Childminding Association (NICMA) surveyed more than 200 newly-registered childminders.
Thirty-five per cent said they knew of at least one unregistered childminder operating in their area.
The organisation said the practise is all "too common and putting children's safety at risk".
The survey also found that 17% of respondents said competition from unregistered childminders was having a negative impact on their business.
NICMA's director, Bridget Nodder, said they are "very concerned at these findings".
"They suggest there is widespread use of unregistered childminders," she said.
"Our main concern is that the use of illegal childcarers who have not been inspected, have not had a criminal records check, and have no insurance is putting the safety of children at risk.
"But the survey findings also show that unregistered childminders are making it more difficult for some legitimate childminders to fill their places.
"It's quite unfair that those who follow the rules and adhere to proper standards are being penalised at the expense of those who are flouting the law."
The charity is launching a publicity drive on Wednesday to highlight the benefits to parents of using registered childminders.
Postcards containing information and advice on choosing a registered childminder will be available at libraries, doctors' surgeries and other public venues throughout Northern Ireland.
The charity is also distributing postcards aimed at unregistered or prospective childminders, explaining the benefits of registering.
Cathy Nelson from Maghera, County Londonderry, has been operating as a registered childminder for 18 months.
She said her business was initially affected by "competition" from illegal childcarers.
"I think a lot of parents don't even know that childminders are supposed to be registered and don't realise unregistered childminders are illegal," she said.
"I think parents need to ask themselves 'What's the best environment for my kids?'.
"It's a very big responsibility looking after a child."
Ms Nodder believes the recession is "tempting" more and more parents to use unregistered childminders who often charge lower fees.
"It's a false economy," she said.
"From what we're hearing, it would seem many unregistered childminders take more children than they would be allowed if they were registered.
"That means they can charge a lower fee but, by looking after so many children, they're compromising the children's safety, and they're unable to provide the individual attention which registered childminders can give."
A spokesperson for the Department of Health, Social Services and Public Safety said: "The DHSSPS would strongly discourage any parent from leaving their children in the care of an unregistered child minder.
"Childminders who look after children in domestic premises, normally the child minder's own home, are required by law to register with their local health and social care trust and have to undergo rigorous checks.
"These checks are designed to ensure the quality of care and the safety of the children who are to be minded are of an acceptable standard."