But Justice Vivien Rose threw out the Prince's argument, opening the way for a full hearing. This risks embarrassing the House of Saud by exposing details of life inside what Harb has described as a royal harem, as well as new claims about their business dealings.
After the ruling, Harb said she was "very happy and relieved".
"If the Prince is going to appeal, I am going to accept the offer of the movie of the book I have written - I am going to spill the beans," she said.
The High Court heard Harb's claim that Fahd, who ruled Saudi Arabia from 1982 until his death in 2005, had promised to provide for her financially for the rest of her life.
She said Abdul Aziz had told her he was prepared to honour the terms of his father's promise and had offered to pay her 12 million and to transfer to her the title of two flats in Cheyne Walk, Chelsea.
Harb said she had taken legal action because she had not received the money or the properties.
Rose said the Prince, who did not attend court, had made no response "as regards the accuracy or otherwise" of Harb's allegations and his submissions had been "limited to contesting the jurisdiction of the court".
The judge said there was no evidence of a similar claim ever being brought in the past.
A letter from the Saudi embassy in London said that the kingdom supported the Prince's claim for state immunity, the court heard.
The judge ruled that the King and the Prince would have been able to claim "sovereign immunity" only if Harb's claim had been brought when the monarch was alive.
A spokesman for Neumans, the law firm which represented Harb, said later: "Her story has received international interest from film producers and she is currently in detailed discussions to progressthe adaptation of her book into afilm."
Harb was born in poverty in Palestine but is now a British citizen.
She claims she married King Fahd when she was 19 and was part of his harem.
She has also claimed that she was not allowed outside the palace walls and was prohibited from having children.
