Alison Spedding is thought to have contracted amoebic dysentery during her imprisonment at the women's prison, in a suburb of the Bolivian capital La Paz. She is suffering from internal bleeding and is to undergo tests and an examination at a hospital in the next few days.
Spedding's father, Ken, will speak about her condition at the House of Lords on Wednesday at the launch of a report by the charity Prisoners Abroad. The report will show that 37 Britons have died in foreign jails in the past five years, many from preventable diseases that have been caused or exacerbated by poor sanitation, lack of medical care and bug- infested food.
Mr Spedding says his daughter's situation is grave. "The last thing anybody wants is for her to die in jail," he said. "We are very worried because although the authorities say it is amoebic dysentery it could blow up into something more serious. Her internal bleeding could be a symptom for anything. They've given her a few pills in prison but they don't know what they're giving them for.
"She has been too weak to get out of bed, which is unlike her. Alison is not the sort of person to get down about her health but she is very depressed - and that gives great cause for concern."
Spedding, now 37, was sentenced last May for possessing and trafficking in marijuana, after a spending a year in the prison waiting for her case to come to trial. Instead of claiming innocence, she admitted the charge of cannabis possession but denies that of trafficking, insisting the 2.5 kilos police found in her flat were for her personal use.
"Alison must be unique among British prisoners abroad," said her mother Maureen Raybold. "Just about all Britons held on drugs charges insist they are innocent but she admits the drugs were there."
But the prosecution said she sold drugs to students, a charge which carried a 25-year sentence. Spedding's family had hoped for a five-year sentence and were given limited encouragement by the judge' summing up, which acknowledged that there "was no evidence of dealing or trafficking". She was sentenced for possession because the amount of marijuana in her flat was more than the quantity permitted for personal use. Her family feel the sentence reflected the pressure the Bolivian government is under from the United States to hand down stiff sentences in all drugs cases.
They also feel, that while the British embassy has been helpful, the British government could do more on Spedding's behalf. They feel that part of her mistake was that she was arrested in a country with no great economic value to the United Kingdom.
"The Foreign Office told me the Government never interferes with the judicial process of another country but Tony Blair certainly did that in the case of the Saudi nurses [Deborah Parry and Lucille McLaughlan] because he was worried that bad headlines were tarnishing arms deals with the Saudis," Mr Spedding said. Her appeal is due to be heard next year, though no date has been set.