A woman who says she is the "secret wife" of the late King Fahd of Saudi Arabia has won a claim topping £20 million after the High Court in London accepted her claim that a son of the king had agreed to a huge payout.

A judge said Palestinian-born Janan Harb, 68, had "a lavish, high-maintenance lifestyle" and plainly received payments in the past "to buy her silence in respect of her relationship with the late king".

But, although her behaviour was "unattractive", the judge ruled, she was "telling the truth" about the agreement and entitled to more than £15 million, plus the value of two apartments in an exclusive part of London worth more than £5 million.

The ruling was against Prince Abdul Aziz - son of another wife of the king - who had denied entering an agreement with Ms Harb when he met her at the Dorchester Hotel in the capital on June 20 2003, while the king was seriously ill.

Ms Harb told the court she had secretly married the king in 1968 when she was 19 and he was still a prince and his country's minister of the interior .

During a hearing over seven days in July, Ms Harb told the court that members of the family of King Fahd, who died in 2005, were opposed to their relationship as she was from a Christian family in Palestine.

Ms Harb said in evidence that, after they separated, the king promised to provide for her financially for the rest of her life.

At the Dorchester in 2003, the prince agreed to pay her £12 million and transfer back to her two flats in Pier House, Cheyne Walk, Chelsea, London, to keep his father's promise of lifelong financial support.

The prince made written statements to the court denying her claim.

But Chancery Division judge Mr Justice Peter Smith ruled her account "credible" and found there had been an agreement.

During the hearing of the case, the judge had ordered Prince Abdul Aziz to attend court in person to give evidence, but was told the current Saudi monarch King Salman and the royal family feared that his appearance would result in "a media circus".

His failure to obey the judge's order led to him being ordered to pay £25,000 to charity for contempt of court.

At the hearing, the judge said the linchpin of the case was whether he believed Ms Harb or the prince, and the best way to decide was to hear the prince give oral evidence and be cross-examined, as Ms Harb had done.

Giving judgment today, the judge said the fact that the prince chose not to give evidence "severely handicapped - fatally so in my view" the attempt by his QC, Ian Mill, to put his case.

After the ruling, Ms Harb said: "This has been 12 years of misery for me. I am very happy with British justice.

"Thank God we have British justice. The prince wanted me to go to Saudi Arabia where he would have had power over all this.

"I am very relieved. I only wish he could have honoured his father's wishes... he is being very mean."

She had told the court in a written statement that King Fahd had been concerned over how their marriage would be viewed by the Saudi public, "many of whom follow a strict interpretation of Islam that preaches deep enmity and hostility to all other religions. It was for this reason that in March 1968 we underwent a discreet ceremony of marriage".

Ms Harb said she converted to Islam shortly before the ceremony.

Over the next three years she fell pregnant three times but each time had an abortion at Fahd's request "because of his concern over the outcry if it had become known that Fahd had fathered a child with a woman from my background".

Ms Harb said she was banished from Saudi Arabia by the king's immediate family in 1970 after they "wrongly" blamed her for his addiction to methadone.

She thought then that banishment had led to a divorce, and she went on to have two further marriages, both of which ended in divorce. The king also married again and had a number of children, including Prince Abdul Aziz.

The judge said Ms Harb had "a lavish lifestyle" and was declared bankrupt in May 2008. Her creditors, according to solicitors to the trustee in bankruptcy, claimed in excess of £3 million.

The amount included "at least £85,000 for gambling and a large amount owed to relatives and friends for living expenses".

The judge said the prince admitted there had been a relationship between Ms Harb and the late king around 1968. But he did not admit the couple had married, although the late king had made "substantial payments" from time to time to Ms Harb.

"In effect she was seeking substantial funds from the defendant as a price for her silence in respect of private matters that affected the late king."

The judge said Saudi diplomat Faez Martini - acting as agent for the "undisclosed principle, namely the late king" - executed a 2001 deed through which Ms Harb was paid £5 million - less the £100,000 deducted as Martini's fee.

In return she agreed to keep confidential information about the late king and had agreed expressly that "she would not herself write or cause to be written her life story, autobiography, diary, memoirs or any other form of information document".

She had also agreed to hand over photographs and other items.

She had used £3 million to pay her debts - and "within the next two years the balance of £1.9 million was also gone".

The judge added: "It is fair to say that she maintained a high maintenance lifestyle as she says to which she had become accustomed whilst being supported by the king."

But it was not in dispute that after the 2001 deed Ms Harb had written two versions of her life story and obtained "considerable sums" for the sale of the book rights.

The judge said: "Whether or not the claim is attractive does not affect the determination of the claim. Either she has a contractual claim or she does not."

He added: "In my view she performed well and maintained the same position with regards the key points of the agreement, despite the firm cross-examination."