A national group that advocates a vegetarian diet is alleging in a lawsuit expected to be filed in Hartford today that McDonald's, Burger King and Friendly's sell grilled chicken products that contain cancer-causing chemicals but hide that information from the public.

The lawsuit, filed by the nonprofit Cancer Project on behalf of two Connecticut residents, seeks to require the chains to post signs in their restaurants and on their menus and advertisements, warning that chicken that is well-done grilled, flame-grilled or barbecued contains chemicals known to cause cancer.

The chemical cited in the lawsuit, known as PhIP, is considered "reasonably anticipated to be a human carcinogen" by the federal government. The federal National Toxicology Program says evidence suggests that eating well-done or grilled meat may be associated with increased cancer risk in humans, but there is insufficient data to conclude the risk comes from PhIP or other specific chemicals.

The lawsuit seeks class-action status to represent people who have eaten, or will eat, grilled chicken from McDonald's, Friendly's or Burger King.

Dan Kinburn, general counsel for the Cancer Project and its affiliate, the Physicians Committee for Responsible Medicine, said the lawsuit is limited to the three chains because the plaintiffs — Mary-Ann Ellison of Meriden and Rebecca Delio of Stamford — had not eaten grilled chicken at other national chain restaurants. The group hopes to find add plaintiffs who have eaten grilled chicken at other national chains, Kinburn said.

PCRM has filed similar lawsuits against fast food chains in California. Earlier this year, the Cancer Project sued hot dog makers in New Jersey in an effort to get warning labels on hot dogs that would tell consumers that eating hot dogs and other processed meats increases the risk of cancer.

A Friendly's spokeswoman said the company does not comment on pending litigation.

Representatives from Burger King and McDonald's did not return calls seeking comment.