RICHMOND, AUG. 24 -- Elizabeth Roxanne Haysom, in an unexpected move on the opening day of her trial, pleaded guilty today to being an accessory before the fact in the murders of her wealthy parents in their Bedford County home two years ago.

The change of plea by the 23-year-old heiress was offered in such a quiet, British-accented voice that Bedford County Circuit Judge William W. Sweeney asked Haysom to repeat it.

Commonwealth's Attorney James W. Updike Jr. said the plea is tantamount to conviction on the two counts of first-degree murder with which she was charged, and carries the same potential penalty of life imprisonment.

Updike said Haysom's court-appointed lawyers told him in advance of Haysom's decision, but added there had been no plea bargain arrangement or agreement on what the sentence should be.

Haysom's plea came at an arraignment moments before the scheduled start of jury selection in the highly publicized murder case, which was to have been the second test of a new Virginia law that permits television and still cameras in the courtroom.

Haysom and her boyfriend, Jens Soering, 20, who were honor students at the University of Virginia at the time of the slayings, were arrested last year in London and charged with the murders of Derek Haysom, 72, and his wife, Nancy, 53.

Soering, the son of a West German diplomat posted in Detroit, is accused of the actual killings. He remains in jail in London, where he is appealing extradition. A British magistrate ruled in June that there was sufficient reason for Soering to stand trial on a capital murder charge in Virginia, but said he was reluctant to extradite anyone who could be sentenced to death because Britain abolished the death penalty about two decades ago.

Despite the guilty plea today, Sweeney required the prosecution to present some of its evidence, including testimony by two Scotland Yard detectives who questioned Haysom and Soering after they were arrested on charges of bank fraud in London last year. The hearing will continue Tuesday.

Prosecutors contend that Soering drove to Boonsboro over Palm Sunday weekend in 1985 to talk to Haysom's parents about their objections to his relationship with their daughter. At Soering's extradition hearing, one of the Scotland Yard detectives who testified today was quoted as saying that Soering told him he "freaked out" during an argument and attacked the Haysoms with a seven-inch knife he had brought with him.

The sheriff's investigation found that Derek Haysom had been slashed 37 times and his wife six times. The killer apparently had attempted to cover up his footprints by swishing his stocking feet on the bloody floor, an action that initially prompted reports that the killings had satanic overtones.

Elizabeth Haysom had gone to Washington in an effort to establish an alibi for Soering, according to testimony at the extradition hearing. After returning from Boonsboro, Soering met her in Washington and they drove back to Charlottesville.

When investigators began to question them, Haysom and Soering dropped out of college and fled overseas. They were convicted in England for bilking a department store out of $9,000 they had received by returning merchandise they had bought with bad checks.