RICHMOND - Antiabortion activists scored a major victory in Virginia as the state's General Assembly agreed Thursday that clinics where most of the state's early-term abortions are performed should be regulated as hospitals instead of as doctors' offices.

Abortion rights advocates, who have fended off similar attempts in Virginia for two decades, say the new rules could be so restrictive that they could force as many as 17 of the state's 21 abortion clinics out of business.

Antiabortion activists said the guidelines are necessary to ensure that the centers are operated safely.

Gov. Robert F. McDonnell (R) has said he will sign the bill. Once enacted July 1, the measure will give the Virginia Board of Health 280 days to write new policies for clinics that perform at least five first-trimester abortions a month. The future rules, detractors fear, could dictate the width of hallways and doorways, impose staffing requirements and require food facilities.

Abortion foes in Virginia and beyond hailed the move as a significant advance.

"It's a historic vote," said Victoria Cobb, president of the Family Foundation of Virginia.

The practical impact of Thursday's vote will rest heavily on the guidelines approved by the board, a 15-member regulatory body dominated by appointees of former governor Timothy M. Kaine (D).

Abortion rights advocates said the regulations could make the state one of the most restrictive in the country for first-trimester abortions. They also said they think that the regulations will place an unconstitutional burden on a woman's ability to get an abortion in Virginia and that they might sue.

"This is a devastating day for the women of Virginia," said Jessica Honke, director of public policy for Planned Parenthood Advocates of Virginia.

She characterized the Virginia measure as one piece of a national effort to curb abortion under revived Republican leadership, including a push in Congress to no longer fund Planned Parenthood.

A number of other states are also considering bills to regulate and license abortion clinics, including Delaware and Georgia.

The bill's passage in Virginia came as the state's Democratic-led Senate voted 20 to 20 Thursday to approve the measure after a long and emotional debate.