A Saudi Arabian princess who had an illegitimate child with a British man has secretly been granted asylum in this country after she claimed she would face the death penalty if she were forced to return home. The young woman, who has been granted anonymity by the courts, won her claim for refugee status after telling a judge that her adulterous affair made her liable to death by stoning.

Her case is one of a small number of claims for asylum brought by citizens of Saudi Arabia which are not openly acknowledged by either government. British diplomats believe that to do so would in effect be to highlight the persecution of women in Saudi Arabia, which would be viewed as open criticism of the House of Saud and lead to embarrassing publicity for both governments.

The woman, who comes from a very wealthy Saudi family, says she met her English boyfriend – who is not a Muslim – during a visit to London. They struck up a relationship.

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She became pregnant the following year and worried that her elderly husband – a member of the Royal Family of Saudi Arabia – had become suspicious of her behaviour, she persuaded him to let her visit the UK again to give birth in secret. She feared for her life if she returned to Saudi Arabia.

She persuaded the court that if she returned to the Gulf state she and her child would be subject to capital punishment under Sharia law – specifically flogging and stoning to death. She was also worried about the possibility of an honour killing.

Since she fled Saudi Arabia, her family and her husband's family have broken off contact with her.

The woman has been granted permanent leave to remain in the UK after the Immigration and Asylum tribunal allowed her appeal.

The Home Office yesterday declined to discuss the case. A spokesman for the Royal Embassy of Saudi Arabia in London said that he would call back but subsequently became unavailable.

Relations between the UK and Saudi Arabia have been strained in recent years and were brought to a head in 2006 when Tony Blair intervened to end a Serious Fraud Office (SFO) inquiry into alleged kickbacks paid in a multibillion military aircraft deal between the two states.

The Saudi royal family was deeply concerned about the idea that the investigators might try o open up their Swiss bank accounts, it was alleged at the time.

This led the Saudis to threaten to restrict the sharing of intelligence relating to terror activity if the prosecution went ahead. They also threatened to pull out of other highly-lucrative arms deals.

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Last year, the House of Lords ruled that the SFO's decision to drop the corruption investigation into the £43bn Saudi arms deal with BAE Systems was unlawful.

In a hard-hitting ruling, two High Court judges described the SFO's decision as "an outrage".

One of them, Lord Justice Moses, said the SFO and the Government had given into "blatant threats" that Saudi intelligence co-operation would end unless the probe into corruption was halted.

"No one, whether within this country or outside, is entitled to interfere with the course of our justice," he said. "It is the failure of government and the defendant to bear that essential principle in mind that justifies the intervention of this court."

The Middle East state has been shrouded in controversy over oppressive policies against women and homosexuals. Secrecy surrounds much of the Saudi legal system, but in a recent report on the use of the death penalty in the kingdom, the human rights group Amnesty International highlighted its extensive use against men and women.

Adulterers face public stonings and floggings and, in the most serious cases, beheadings and hangings.

The high numbers of executions in Saudi Arabia in 2007 continued into 2008. There were at least 102 executions of men and women last year – at an average rate of two every week. Amnesty is aware of at least 136 individuals currently awaiting execution.

Last week, Saudi Arabia's religious police were blamed for the death of two sisters who were murdered in what was deemed an "honour killing" by their brother, after the sisters were arrested for allegedly mixing with men to whom they were not related.

The Society for Defending Women's Rights in Saudi Arabia said that the religious police had arrested the two sisters, aged 19 and 21, thus putting their lives in danger.

Their brother shot them dead in front of their father when they left a women's shelter in Riyadh on 5 July, according to Saudi news reports.

In 2007, in a case that shocked Saudis, a woman from Qatif was sentenced to 200 lashes and six months in prison after being gang-raped. She offended cultural expectations because she was unaccompanied when she got into a car with a former boyfriend.

The man had agreed to hand back a photograph of the woman who was about to marry another man, but as they drove along a street they were stopped and seized by seven men who raped them both. The woman was originally sentenced to 90 lashes but the sentence was increased when she appealed. Eventually, after an international outcry, she was pardoned.

In 2007, King Abdullah II of Saudi Arabia was jeered during a state visit to Britain as dozens of demonstrators turned out to protest at his country's human rights record.