RICHMOND — Attorney General Ken Cuccinelli II has advised a state board that it cannot impose new regulations that some argue would for the first time allow gay couples to adopt children in Virginia.

Cuccinelli’s office said in a memo dated Tuesday that the proposal to be considered by the State Board of Social Services as early as next week “does not comport with applicable state law and public policy.”

“Therefore the State Board lacks the authority to adopt this proposed language,’’ wrote Allen Wilson, senior assistant attorney general.

Cuccinelli’s position reverses one of his predecessor, William C. Mims, a former Republican legislator and now a Virginia Supreme Court justice.

Currently, only married couples and single men and women — regardless of sexual orientation — can adopt in Virginia. Proposed changes would require private and faith-based groups, such as Catholic Charities or Jewish Family Services, to allow gay parents to adopt or foster children. But there’s disagreement between lawmakers and activists about whether the proposed regulations would allow other classes, including unmarried couples — heterosexual or homosexual — to adopt.

Del. Robert G. Marshall (R-Prince William), who has been lobbying Gov. Robert F. McDonnell (R) to do what he can to kill the proposal because he does not think it is healthy for gay couples to raise children, said he was pleased by the advice.

“It was a correct decision in my judgment,’’ he said. “The General Assembly has not made sexual orientation a protected class. This will be welcome news to a lot of faith- based adoption agencies.”

Cuccinelli (R) angered gay-rights activists last year when he urged the state’s public colleges and universities to rescind policies that ban discrimination on the basis of sexual orientation, saying they had no legal authority to adopt such statements.

In December 2009, the attorney general’s office, then headed by Mims, advised the state board that it had the authority to repeal the existing regulation and adopt the new rules. Cuccinelli’s office revised that advice this week.

“His memo at this stage is advice, and clients get to say one way or another whether they accept the counsel,’’ said Claire Guthrie Gastanaga, general counsel to Equality Virginia and a former chief deputy attorney general. “He was wrong when he said the state college boards of visitors did not have the authority to implement equal-opportunity policies. He’s equally wrong here.”

Former Democratic governor Timothy M. Kaine, who is running for U.S. Senate next year, proposed the change to the regulations in November 2009, less than two months before he left the office to become chairman of the Democratic National Committee.

McDonnell has repeatedly said that he opposes the regulations because, he believes, faith-based organizations should be able to make their own policies. He said he would review the actions of the state board, a nine-member panel. All but four members are holdovers from Kaine’s administration.

Several gay-rights organizations held news conferences in Northern Virginia and Richmond on Wednesday to lobby McDonnell to eliminate discrimination in the adoption process.