HELENA — Health insurance companies must include coverage for prescription contraceptives such as birth control pills in prescription drug plans, and failing to do so constitutes sex discrimination, Montana Attorney General Mike McGrath said Tuesday.

McGrath, in a legal opinion sought late last year by state Senate President Jon Tester, now a Democratic candidate for the U.S. Senate, said Montana’s ‘‘unisex’’ insurance law forbids any discrimination based on gender in insurance policies. His opinion carries the weight of law unless overturned by a court or modified by the Legislature.

Attempts by women’s advocacy groups, like NARAL Pro-Choice Montana, have failed in recent years to get lawmakers to force insurance companies to automatically offer the coverage.

Blue Cross Blue Shield of Montana said company lawyers are still reviewing the decision and have not yet determined when the mandate will become effective in current prescription drug policies. The company has not decided if it will appeal McGrath’s decision.

In addition to the state’s unisex insurance law, McGrath also cited a state Supreme Court case finding it illegal to omit pregnancy coverage from insurance policies.

McGrath held that ‘‘it follows that exclusion of coverage for prescription contraceptives and related medical services would also be sex discrimination and a violation of Montana’s unisex insurance statute.’’

Insurance companies have argued that so-called mandated benefits force them to charge more to all customers, even those who have no interest in such coverage. Others argue that any increase would be minimal, and easily offset by preventing the high costs of unwanted pregnancies.

Tester lauded the opinion, saying it ‘‘is a victory for women and equality.’’ He said Montanans ‘‘consistently support common-sense policies that protect and improve women’s health.’’

A bill backed by Tester in the 2005 Legislature that would have mandated the coverage cleared the Senate but died in the House.

NARAL spokeswoman Allyson Hagen called McGrath’s decision a victory for women’s health care.

‘‘McGrath’s opinion means that insurance companies can no longer cover Viagra but then refuse to pay for birth control,’’ she said in a release. ‘‘Today’s decision marks an end to a loophole that allowed insurance companies to discriminate against women in prescription drug coverage.’’

Blue Cross Blue Shield of Montana has pointed out that other mandated insurance benefits from the 2003 Legislature alone already add about $15.50 to each insured person’s monthly premium. More mandates could further drive up the cost, the company said.

‘‘The primary reaction is, while there are a lot of services that are very good for Montanans, what is covered in an insurance policy should be the choice of the individual paying for the policy,’’ said Tanya Ask, Blue Cross vice president of government affairs. ‘‘Any time you are mandating coverage, you are overall increasing the cost of the coverage.’’

Blue Cross argues employers should be able to choose whether to include, and pay for, optional contraceptive coverage in the policies they offer employees.