The customs officer grinned: 'You will be hung!' He pulled 5.6kg of heroin from my bag

July 1997, Karachi Airport, and Rosemarie Morley was eager to get home. She had been on a short trip with her boyfriend Markus and they were keener perhaps than most travellers to get through the formalities quickly. The two of them joked with the Pakistani customs officials - until their cases were taken away for examination. As the minutes stretched out, it became clear that something was very wrong. Then out came a grinning customs man. "You will be in prison for life! You will be hung!" he smirked. Rose burst into hysterical tears. The customs officials had found heroin, a massive 5.6kg of it, hidden in their luggage. Rose and Markus weren't going home for some time.

A combination of extraordinary naivety and chronic lack of cash landed Rose and Markus in the filthy, rat-infested prison barracks where they were to spend the next two years. They have maintained ever since that they had no idea that their bags were loaded up with heroin - although they had foolishly agreed to an "errand" for a friend of a friend that they knew involved carrying contraband. But whether she knew what she was doing or not, Rose Morley is an unlikely candidate to end up in a Pakistani women's jail. She had never done anything wrong before; and she had been brought up in a very happy, close-knit family.

Conditions were appalling. "Everybody was in this huge barracks, there were open bars to the outside," says Rose, who was released 10 days ago. "The beds were metal and raffia, and when you got in, until somebody left and gave you a bed you slept on the concrete floor with the rats. There was no sanitation - there were three holes in the floor for the whole place." Markus was in the neighbouring men's prison. It was built for less than 1,000 prisoners and it held over 5,000. "There was heroin in there, people with guns - he was dodging bullets, we could hear the shooting."

Constantly ill and suffering from depression, for days on end Rose was unable to wash or dress. She felt as though her life was finished, as though the sentence would never end. Her diaries are a frantic stream- of-consciousness. One entry from August 1997 records her hallucinations with a temperature of 103; she thought she saw a ghost. Another reads: "Mark is so ill, worrying me, he seems so weak now" - the two saw each other regularly in court, where Markus would be brought in in chains.

Meeting Rose, it's hard to believe that she has only just come back from such a place. Now 32, she looks tanned and strikingly pretty, almost as though she's just returned from holiday. But when she speaks of the past two years tears well up frequently as the words tumble out.

So how did she end up in Karachi Central Prison? Before her ill-fated trip, Rose had been approached by an acquaintance in a north London pub and asked if she would go to Pakistan to pick something up. She was offered pounds 3,000 to make the trip. Rose claims she believed she was being asked to collect a set of fake financial bonds. She knew this was illegal, but thought it was worth the risk. Money was tight; not least because she was bringing up her son Matthew, then seven, single-handed and at the age of 30, when most people have a career well under way, she had never managed to stick at a single long-term job. "I was a bit of a perpetual student - I did little jobs that would earn me money without tying me down to nine to five, I'm not that sort of person," she says.

Rose had no qualms about leaving Matthew with her parents and slipping off to Pakistan without telling them she was going - she expected to be back within a few days, her family none the wiser. But there are dozens of horror stories about the penalties for criminal activities in Asian countries - and surely anyone asked to bring anything back from this part of the world would immediately suspect some kind of drug connection?

"I was desperate for money," she shrugs. "My bills were piling up, everything was about to be cut off. Other people that I know had gone and come back with no problem. I was told nothing else could possibly pass through the x-ray machine. I thought, well, why not? A part of me was suspicious but I didn't want to be suspicious because I needed the money." She has since discovered that collecting "bonds" is a common dupe for unwitting drugs carriers.

In Pakistan, Rose and her boyfriend's suitcases were taken away from them and re-packed. Rose opened her case to see what was there and found nothing untoward; the heroin had been secreted in the case's handle and walls. When they were stopped at Karachi airport, it took the customs officials several hours to prise it out of its hiding places. "I was convinced I'd be able to tell everyone I didn't know what it was and go straight home," recalls Rose. Initially the couple pleaded not guilty but after seven months waiting for trial they decided to change their plea, believing a short prison sentence was preferable to the strain and expense of many years of legal wrangling.

Rose was sentenced to three years and nine months in the women's' wing of Karachi Central Prison. What she hadn't realised, however, was that under a new Pakistani law, anyone convicted of drug smuggling could also be charged with possession of drugs - an offence which can carry the death sentence, or life in prison. After Rose and Markus had been found guilty, the second charge was filed against them. Rose was determined to fight back. "I knew that it was against the principles of international law to charge someone twice for the same crime."

She began writing to everyone she could think of, human rights organisations, British lawyers, the Queen, Tony Blair, drawing attention to prisoners in Pakistan. British diplomats and lawyers were already discussing the legal anomaly with their Pakistani counterparts, but somebody had to be the test case, and Rose and Markus were designated to appear in the local High Court.

At home, Rose's mother collapsed in shock when Rose was arrested. Then the family rallied round to help. One of her sisters raised money, the other took on Rose's little boy. The family decided to tell Matthew that his mother was in hospital rather than in prison - he only learned the truth from a newspaper report of her case last year. Taped in her diary is a note from him: "To mummy. I love you lots. I had a lovely birthday but I didn't have it all that good because you weren't there. Love Matthew." She couldn't bear even to see pictures of him. "I put them away because I didn't dare look at them. It was as if I didn't have a son for two years. I had to be strong, I couldn't afford to crack up in there." Her parents, she says, have been "fantastic. They haven't judged me at all".

Rose was moved to the only

individual cell in the barracks seven months into her sentence; as an educated Class B prisoner she was entitled to what privileges were available. She learnt basic Urdu. She was given rations of flour, ghee and dahl and gave them away to other women in return for their cooking skills. "The kitchen was a war-zone because of the rivalry between different prisoners. I wasn't going in there. I bought food from outside too - we were allowed to shop once a fortnight."

The support group Prisoners Abroad sent her pounds 60 a quarter, which was topped up by her family, to buy food, paper and pens and clothes. She also worked as an English teacher for the children in the prison, which earned her a remission on her sentence.

After months of legal to-ing and fro-ing, Rose and Markus were acquitted of the second charge of possession by the High Court on 13 May this year. They were released on 19 May, her birthday, and flew home the same day. Solicitor Stephen Jakobi, founder of Fair Trials Abroad, who campaigned on Rose's behalf, says that her case may make all the difference to others charged twice over drugs offences - several dozen Europeans, several hundred non-Pakistanis and as many as 2,000 native Pakistanis.

"This is a key case," he says. "Exploited properly it could result in the biggest mass release of people in a criminal situation since World War II. The result should be over 2,000 people at liberty and free to go home." This, he says, is largely due to Rose's efforts. "She was the activist, sitting in prison, gathering information and persuading other Europeans in similar situations to write to us."

Mr Jakobi only works for innocent people who have found themselves on the wrong end of a miscarriage of justice, and those charged for a second time for the same offence most definitely fall within this brief. "It is contrary to international law, contrary to the policies of the International Committee for Civil and Political Rights, and it is also against the Pakistani constitution. Everyone knows that once you've been acquitted you can't be tried again for the same crime, but they don't realise that equally, once you've been found guilty, you can't be tried again."

Rose hopes that the success of her case will lead to friends she left behind in the Karachi prison also finding their way home. She does not try to make excuses for what she did. But, she insists, she is far more naive than criminal. "I did know I was breaking the law, and I think I was caught for a reason - to teach me a few things about myself. I've learned a lot of lessons." She plans to do a course for university entrance next year, and do voluntary work for groups like Prisoners Abroad or for abused women in the meantime. She wants other people to know her story so they don't make the same mistakes. "I wouldn't wish what I've been through on anyone else. Not even on my worst enemy."