Azerbaijan family of Margaret Thatcher"s one-time interpreter plead for her release

She was also targeted for helping families whose homes were destroyed to make way for buildings for the 2012 Eurovision Song Contest in Baku, the capital.

Ilham Aliyev, Azerbaijan's hardline president, stands accused of launching an unprecedented crackdown on dissent that has intensified since the beginning of this year, jailing dozens of rights campaigners, lawyers and journalists.

Human Rights Watch describes the charges against Mr and Mrs Yunus as "completely bogus, and intended to silence them". The Azerbaijan government denies the charges are politically motivated.

Mrs Yunus is a former deputy defence minister and skilled linguist who met Lady Thatcher when she visited Baku in 1992.

Lady Thatcher, who had resigned as Prime Minister two years earlier, travelled to newly-independent Azerbaijan to hand the Azeris two cheques worth $30m on behalf of BP that was a down payment on Caspian oilfields. That agreement paved the way for the "Contract of the Century" signed in 1994, which saw BP take the lead in a consortium extracting hydrocarbons from the seabed.

During Lady Thatcher's visit, Mrs Yunus was given the role of escorting her, helping translate and explaining the war that was then raging between Azerbaijan and Armenia in Nagorno-Karabakh. "They were both strong women and they got on well," said her brother-in-law.

^{Leyla and Arif Yunus in 2013}

Mrs Yunus, who is now being held in a pretrial detention centre, has diabetes, hepatitis and eye problems that her family believe could be exacerbated in custody. Her lawyers say she has been subjected to verbal and physical abuse by a senior guard and a fellow inmate in her cell.

Arif Yunus, who suffered a stroke earlier this year, is in detention at a facility run by the national security ministry, which is notorious for torture of inmates.

Analysts believe Mr Aliyev is using the West's distraction by events in Ukraine and the Middle East to liquidate all opposition ahead of the inaugural European Games in Baku next year, which will be sponsored by BP.

Ramis Yunus, a former chief of staff of the Azeri government who lives in Baltimore in the United States, said BP should confront Mr Aliyev and withdraw its sponsorship from the Games. "This event is being used by the regime to improve its image and cover up its repression," he said.

Dinara Yunus, 29, the daughter of Leyla and Arif Yunus, fled Azerbaijan for Holland in 2009 after her parents received threats that she would be harmed.

Speaking by telephone from Amsterdam, she said: "Are economic interests more important than human rights? By telling the government of Azerbaijan to do something about political prisoners, BP could raise its image and be more ethical.

"BP should call for the immediate and unconditional release of the political prisoners, including my parents. BP is powerful enough to set these rules."

The campaign against dissent in Azerbaijan peaked this summer. In the most prominent cases, Rasul Jafarov, a young rights activist, was arrested on tax evasion charges on August 2, and Intigam Aliyev, a veteran lawyer who had filed complaints about election fraud to the European Court of Human Rights, was detained on similar charges a few days later.

Later the same month, Ilgar Nasibov, a journalist in the Nakhichevan region, was left unconscious with his face beaten to a bloody pulp, after an attack by unknown assailants. He had investigated a controversial case involving a death in custody.

In a statement to The Telegraph, BP said that it had "a strong and long-standing commitment to respecting the human rights and dignity of all people, wherever we do business".

However, the company said it believed "the government of Azerbaijan has the primary responsibility to protect human rights and we remain ready to implement their guidance in this regard".

It declined to comment on whether it would withdraw sponsorship of the European Games.