Almas Guliyeva was stopped at Baku airport

Almas Guliyeva, 46, from Golders Green, north London, is married to the nephew of the chairman of an opposition party in the former Soviet Republic.

She was held at Baku airport on Friday after attending a funeral.

Her son Ilkin Guliyev, 19, said she was so distraught at being arrested she had suffered a heart attack.

I dread to think what she is going through Ilkin Guliyev

He told BBC News his mother was now in a serious condition in hospital in Baku.

He claimed a judge had ruled she would be placed under house arrest once she had been discharged.

The Foreign Office confirmed Mrs Guliyeva, who lives with her two sons, had been arrested, had been taken ill while in custody and was now in hospital.

The spokesman said a consular official was due to visit her on Tuesday and would "offer appropriate consular assistance".

He said it had not been established if she was going to be charged.

Customs officers found a pistol in her luggage as she tried to board a plane back to London, the authorities in Azerbaijan said.

But her son said: "There is no way she would have done this. She is a mother-of-two. She has a nine-year-old. Why would she risk her life?"

'Politically motivated'

He claims the gun was found when her luggage was searched for a second time. He believes instead that her arrest was politically motivated.

Consular officials are due to visit Mrs Guliyeva

Mr Guliyev's great uncle, New York-based Rasul Guliyev, is the chairman of the Democratic Party of Azerbaijan and a former speaker of Azerbaijan's parliament.

"My great uncle is outspoken of the government. He challenged the president for the election in 1998."

He said since then the authorities had arrested other members of his family.

"My mum doesn't get involved. She is British and thought it was OK to go over there because my uncle had died."

Mr Guliyev said his father, who had been away on business in Russia, was now home but not coping well.

He has not yet spoken to his mother but is being kept informed of the situation by relatives.

He is concerned that she gets the medical treatment she needs.

"She is not a strong woman. She has had heart problems in London. I dread to think what she is going through.

"I am keeping busy but in the evenings when I am home it keeps going through my head what is she going through?

"I can't see her. I can't talk to her."

He is now gathering names on a petition and intends to protest outside Downing Street on Tuesday evening.

Azerbaijani newspaper Muxalifat has already reported Mrs Guliyeva's lawyer as saying she had nothing to do with the pistol.