(Reuters) - The United States government on Monday overturned its 30-year ban on blood donations by gay men, saying they can now donate 12 months after their last sexual contact with another man.

The Food and Drug Administration said its decision to reverse the policy was based on an examination of the latest science which shows that an indefinite ban is not necessary to prevent transmission of HIV, the virus that causes AIDS.

"Ultimately, the 12-month deferral window is supported by the best available scientific evidence, at this point in time, relevant to the U.S. population," Dr. Peter Marks, deputy director of the FDA's biologics division, said in a statement.

The move brings the United States in line with countries such as the United Kingdom, Australia and New Zealand which also have 12-month deferrment periods.

Gay rights advocates said the updated policy reamins discriminatory.

"It is ridiculous and counter to the public health that a married gay man in a monogamous relationship can't give blood, but a promiscuous straight man who has had hundreds of opposite sex partners in the last year can," said Jared Polis, a Democratic congressman and co-chair of the Congressional LGBT Equality Caucus, a caucus of openly gay members of Congress.

The FDA said it has worked with other government agencies and considered input from outside advisory bodies, and has "carefully examined the most recent available scientific evidence to support the current policy revision."

During Australia's switch from an indefinite blood donor deferral policy on gay men, essentially a ban, to a 12-month deferral, studies evaluating more than 8 million units of donated blood were performed using a national blood surveillance system, the FDA said.

"These published studies document no change in risk to the blood supply with use of the 12-month deferral," the agency said. "Similar data are not available for shorter deferral intervals."

Additionally, the agency said people with hemophilia and related blood clotting disorders will continue to be banned from donating blood due to potential harm they could suffer from large needles. Previously they were banned due to an increased risk of HIV transmission.

The agency said it has put in place a safety monitoring system for the blood supply which it expects to provide "critical information" to help inform future FDA blood donor policies.

The FDA said its policies have helped reduce HIV transmission rates from blood transfusions from 1 in 2,500 to 1 in 1.47 million.

The FDA first proposed the changes in May. It received some 700 public comments. About half recommended keeping the ban in place.