The use of morning-after pills by American women has more than doubled in recent years, driven largely by rising rates of use among women in their early 20s, according to new federal data released Thursday.

The finding is likely to add to the public debate over rules issued by the Obama administration under the new health care law that require most employers to provide free coverage of birth control, including morning-after pills, to females covered by their plans. Some religious institutions and some employers have objected to the requirement and filed lawsuits to block its enforcement.

Morning-after pills, which help prevent pregnancy after sex, were used by 11 percent of sexually active women from 2006 to 2010, the period of the study. That was up from just 4 percent in 2002. Nearly one in four women between the ages of 20 and 24 who had ever had sex have used the pill at some point, the data show.

Morning-after pills are particularly controversial among some conservative groups who contend they can cause abortions by interfering with the implantation of a fertilized egg that the groups regard as a person.