Shirlee Cook, who also claims to have been beaten by police, pleaded guilty to secure her release but denies the possession charge.
Mrs Cook, a housewife from South Ealing, in west London, believes the payments were a fraud involving the local police officers who should not have needed paying to release her. The maximum fine laid down under Thai law for a minor drugs offence is stated to be about pounds 250.
She insists that she was "framed" by police and is furious at having spent five days in a cell at the end of her holiday at Patong Beach, in northern Thailand. Her husband, Brian, had to sell his car to make the payments which secured her freedom.
"I'm so angry, really angry that I had to go through all that," Mrs Cook said. "To think that a pounds 25 fine cost us more than pounds 5,000. I'd say to other people not to go to Thailand. It's really not worth it."
Airtours, the tour firm which arranged the couple's holiday, said its representative was told by Thai police that Mrs Cook would be released only if their request for more than pounds 5,000 was met. "We acted in good faith," the spokeswoman said.
The Airtours representative, Anna Roberts, accompanied Mr Cook to the police station on the morning after Mrs Cook's arrest. She wrote out details of the police demand for 200,000 baht - pounds 5,063 at the then exchange rate - before Mr Cook flew home to Britain to arrange the transfer. Mrs Cook was freed later that week.
Airtours said it understood that most of the money was the fine, although it had charged small sums for food and blankets which Ms Roberts provided for Mrs Cook.
Details of payments to "police, lawyer and immigration" were confirmed in writing to her by the company on her return to Britain.
But she was never given a receipt for what was paid in court. She claimed to have seen two 500 baht notes handed over in court by the man she now understands to have been her legal representative but had no proof that this was the fine.
However, a translation of a notice in her passport has now apparently confirmed her suspicion. It read: "On the 10 November 1995, Mrs Shirlee Ann Cook, British national, aged 55 years, was accused of unlawful possession of drugs. The court of Phuket convicted and fined the fee of 1,000 baht . . . [around pounds 25]".
Mrs Cook wants her money returned. She said: "I feel really bad that Brian had to sell his car to get me out." And she added: "I want to know what happened. If I can stop one other person from going through what I did, then I want to."
Both Thai and British officials have launched investigations. A Foreign Office spokes- man said that it had taken very careful note of what she had to say and its embassy in Thailand was investigating. It had heard of no similar cases to Mrs Cook's.
Piyavaj Niyomrerks, of the Thai embassy in London, said officials there were waiting for more details from Mrs Cook to commence an investigation in Thailand.
"This is most serious and we are quite concerned," he said, adding that the authorities would not tolerate any bad behaviour from people in positions of authority as it would tarnish the image of the country as a tourist destination. "This is the first time the embassy has learnt of this kind of incident," he said.
A spokeswoman for Airtours, one of Britain's biggest travel companies, said it had been a third party to the case and its representative had worked very hard to help Mrs Cook.
"This is obviously a very serious and distressing incident that Airtours have done everything to assist Mrs Cook and her husband in. The situation is that at the time, Airtours acted in good faith," she said.
"We only ever paid money to police of amounts that were specifically requested by them. We were informed that if the money was not paid Mrs Cook would not be released.
"No payments were paid without our client's knowledge. However, if she does wish to pursue it, it would be a matter for her to take up directly with the Thai authorities."
There was no indication whatsoever that the money was a bribe, the spokeswoman said, adding that the company had met Mrs Cook's legal costs and paid for accommodation after her release. "We negotiated on her behalf and assisted her, but at the end of the day, the decisions and fines imposed are not something that Airtours has any control or influence over."