On the eve of the Obama administration's new health care law coming into effect, big health insurance companies have decided to stop selling policies for children citing potentially large unexpected costs and uncertainty in the health insurance market.

The new health care law to take effect on Thursday states that health insurance plans must not be denied to children less than 19 years of age with pre-existing medical conditions. The same provision goes into effect for adults in 2014. Anthem Blue Cross, Aetna Inc. and others will halt new child-only policies in California, Illinois, Florida, Connecticut and elsewhere as early as Thursday.

"Unfortunately, this has created an un-level competitive environment," Anthem Blue Cross, California's largest for-profit insurer, said in a statement declaring its intention to "suspend the sale of child-only policies" on Thursday, six months after the healthcare overhaul was signed.

The move has angered lawmakers, regulators and healthcare advocates. "It's obviously very unfortunate that insurance companies continue to make decisions on the backs of children and families that need their help," White House Press Secretary Robert Gibbs said at a news briefing.