Salah Hejailan said that if Yvonne Gilford's relatives agreed then the accused pair could escape with three or four years' jail - possibly suspended.
Mr Hejailan, who will visit Deborah Parry and Lucille McLauchlan in jail for the first time on Saturday, said that if the victim's family declared they did not want the ultimate sanction this would rule out capital punishment.
Speaking from the Saudi capital Riyadh, he told The Independent: "Everything depends on the family of the dead woman. The judge would not even consider capital punishment unless it was asked for by relatives."
He would be seeking the assurance from the Gilford family in writing, he said, which could mean the entire case being dealt with inside a few weeks.
It would be "shocking", and not a little ironic, said Mr Hejailan, for a Western family to urge the death penalty at a time of unbalanced criticism of the Saudi justice system. The Saudi insistence on taking into account a victim's views was a positive factor, he said.
The dead woman's brother Frank Gilford, speaking from his home in Sydney, Australia, hinted at the weekend that the family might not ask for execution for her alleged killers.
Mr Hejailan added he was aware of reports that at least one of the two accused, Ms Parry, 41, from the Midlands, had withdrawn an alleged confession, but he said he was unable to comment in detail on the strength of their defence until he met them.
Saudi newspapers have quoted police sources as saying the three women, who worked together at the King Fahd Military Medical Complex in Dhahran, had rowed before the killing, with the two Britons allegedly taunting the 55-year-old Australian over her age.
The same sources claim that the two women were caught after police discovered that money was disappearing from the dead woman's bank account and followed them to a cash machine. The women were visited on Monday by the British consul, Tim Lamb, who said they were in "good health" but naturally very concerned about the murder charges.
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