A group of Dallas-area families have filed a lawsuit seeking to block Gov. Rick Perry's executive order to vaccinate Texas schoolgirls against the virus that causes cervical cancer.

Perry, a Republican, has defended the order as critical to saving lives. But it has inflamed conservatives who say it contradicts Texas' abstinence-only sexual education policies and intrudes into family lives. It also upset many state lawmakers in his own party and a bill to override the order is working its way through the state House of Representatives.

Perry wants to require the vaccine, called Gardasil, for girls entering sixth grade as of September 2008. It protects girls and women against strands of human papapillomavirus, or HPV, that cause most cases of cervical cancer and genital warts.

Kenneth Chaiken, the attorney representing the families, said he believes it is the first lawsuit filed by families with children who would get the vaccine. He said the families don't believe Perry has the legal authority to issue such an order.

"They are families who very significantly object," Chaiken said.

The lawsuit was filed in Travis County on Thursday.

Perry spokeswoman Krista Moody said the governor is confident he had the authority to issue the order.

"He takes his job as the leader o the state of Texas very serious. He sees this vaccine as not only a fiscally responsible order but also one that has the potential to save the lives of thousands of women in Texas," Moody said.