A Virginia legislative panel dominated by defense lawyers, which for four years has blocked reform of the state's sexual assault laws, today approved sweeping changes in the laws but added an amendment that proponents say could reverse the bill's intent.

Designed to encourage victims to report cases of alleged rape by shielding their sexual histories from courtroom scrutiny, the amended bill now says such evidence may be introduced at the discretion of a judge if it is found to be necessary under the state's constitution.

The amendment will aid those accused of rape by permitting them greater leeway in attempting to discredit the testimony of their accusers. It was tacked onto the bill by the powerful House Courts of Justice Committee in the face of mounting pressure for rape law reform from state prosecutors, law enforcement officials and women's groups.

"What this bill does now is override present case law and open up a woman's sexual history," said Stafford County Commonwealth's Attorney Dan Chichester, who said the amendment gives him "serious misgivings" and could endanger the tenuous coalition of groups that has been pushing for the change in the law.

"As a practical matter, we're going to have evidence rules that vary from judge to judge, and it's going to be awhile before we know what the state of the law is."

When Del. Theodore V. Morrison (D-Newport News) orchestrated the reform measure's death last year, the bill's patron, State Sen. Frederick C. Boucher (D-Washington), was vehement in criticizing the all-lawyer panel's tendency to favor defendants -- and defense lawyers -- over victims. Today Boucher claimed the bill's emergency from committee as a major victory and said the amendment, a compromise measure that was attached after he and Morrison tangled in a lengthy and emotionally charged debate, served only to codify existing law.

"I'm extremely pleased," Boucher said. "I think our four years of work on this have been very well justified, and I don't regret a single minute I've spent on it."

Despite the squabble over the evidence provision, panel members left intact several other sections aimed at providing further protection for the rights of rape victims as the measure headed for what was expected to be a victory of the floor of the House.

Among them were provisions that a rape victim need not prove that she physically resisted her attacker in order to win a conviction, that defense attorneys may not introduce general reputation evidence on the victim's previous lack of chastity, and that a husband may be prosecuted for the rape of his estranged wife when the woman can furnish written proof that the couple has been living apart with the intention of staying apart.

Eliminated in an earlier subcommittee compromise was a provision that held the victim's testimony need not be supported by other evidence to sustain a rape charge.

The bill, which does not limit the crime of rape to male defendants, specifies that a rapist who attacks a stranger could go to prison for life, while the penalty imnposed on a husband who attacks his estranged wife is limited to 20 years. Rape performed with an object would carry a maximum sentence of life in prison.

Citing FBI statistics that show rape as the most underreported of all crimes, law enforcement officials and women's groups around the state have been advocating a revision of the law for years in an attempt to encourage women to report such crimes without fear of public embarrassment. Under current federal statistics, reported sexual assaults increased 66 percent between 1969 and 1978.

Morrison, one of the House's most influential Democrats and a prominent defense lawyer, had undertaken a one-man campaign to revise the rape measure, exacting concession after concession from proponents who felt his support on the Courts of Justice Committee was critical to the measure's survival.

While Morrison and Boucher battled over the evidence question, at least one member of the panel appeared unconcerned about the measure's fate. Speaker of the House Thomas W. Moss (D-Norfolk) laughed and joked repeatedly during the hearing, twitting Boucher for his willingness to speak to the press and remarking often that he was bound to vote for the bill because "my city editor told me to."