Tanya Davis fled Jizhou Number 1 middle school one winter morning in March across cotton fields, still covered with the stubble from last autumn’s crop.

In the nine months during which she and her boyfriend taught English at the school, in rural north China, they endured extra working hours, unpaid wages and severe temperatures without heating. With hearts pounding and worried their employer would find a pretext to stop them leaving, they lugged their belongings past a sleeping guard and into a taxi.

“The sense of relief was immense,” said Ms Davis, 23, from Wales. “I felt like we had crossed our last hurdle and everything was going to be OK.”

Their problem is not a one-off. Britons and other foreigners drawn by China’s growth and hunger for English lessons are increasingly finding themselves in educational sweatshops.

In one case, an American man lost his life. Darren Russell, 35, from Calabasas, California, died in mysterious circumstances days after a dispute in which he quit his teaching job in the southern city of Guangzhou. “I’m so scared. I need to get out of here,” Mr Russell said in a phone message to his father hours before his death in what Chinese authorities said was a road accident.

As China opens up to the world, public and private English-language schools are proliferating. Most treat foreign teachers well, and monthly wages can run to pound;526 plus board, lodging and airfare home. But complaints about bad experiences are increasing.

The British embassy in Beijing warns on its website about breaches of contract, unpaid salaries and broken promises. The US Embassy says complaints have increased eight-fold since 2004 to an average of two per week.

Foreign teachers in South Korea, Japan and other countries have had similar problems, but the number of allegations in China is higher because “the rule of law is still not firmly in place”, said a US embassy official who did not want to be named.

Of the late Mr Russell’s case, the US House of Representatives international relations committee said in a recent report: “A number of substandard English language teaching mills have sprung up, seeking to maximise profits while minimising services.”

It said some of these institutions had become “sweatshops” where young, often naive, foreign teachers were held as “virtual indentured servants”.

Ms Davis said officials at her school in Hebei province piled on extra classes with no extra payment and deducted sums from her pound;329 monthly salary for random taxes and phone calls that were not made. “The more we let them get away with, the more they tried to get away with,” she said.

Numbers are hard to track. The education ministry said there was no record of how many language schools exist because local governments administer them. Education officials in Beijing, Guangzhou and Shanghai - China’s major cities - did not respond to telephone and fax requests for information.

China now has a huge demand for English teaching, spurred by its emergence as an economic power and the approach of the 2008 Olympic Games to be held in Beijing. “The market is huge,” said Frank Dong, 38, manager of the American TESOL Institute in Beijing, which gives contracts to about 100 teachers a year from outside China. “There is now a tremendous internal need that drives Chinese people to improve their English.”

Wages range from pound;150 to pound;500 a month for an average of 20 hours per week, plus overtime. Housing is usually provided and many schools promise about pound;500 towards the airfare home upon completion of a one-year contract.

Many jobs are advertised on the internet. On one of the most popular sites, Dave’s ESL Cafe, more than 340 were posted over three months. But on the same site there is an anonymous warning from a teacher about a school in the south of China.

“They will use you, abuse you, cheat you and disrespect you,” it says. “You will hear it all when they want you to sign the contract. Then after it’s, ‘Oh sorry, that isn’t in your contract.’ Or a bunch of excuses that go on and on.” There is no standard rule on contracts. Some are in English, some in Chinese.

Mr Russell had a disagreement with the manager of Decai language school, in Guangzhou, where he had been promised 20 hours of classes per week.

Instead, he had to teach at two schools, where he put in up to 14 hours a day and oversaw 1,200 students, his mother, Maxine Russell, said.

The school had experienced problems with foreign teachers. Two had quit by the time Mr Russell showed up, and a former Decai employee, a Chinese woman who did not want to be named, said she left because she was asked to recruit foreign teachers by offering attractive contracts.

In April 2005, ill with bronchitis and exhausted from long hours, Mr Russell told manager Luo Deyi that he wanted her to lighten his workload. A row ensued, Mr Russell resigned and threatened to tell the police that Luo was operating illegally, the former employee said. The school then moved him into a low-budget hotel. A week later he was dead.

Police told Decai and Mr Russell’s mother that he had been killed in a hit-and-run traffic accident. Mr Russell’s body was later shipped to California. Mrs Russell said Chinese authorities could not provide consistent witnesses or a time of death. But Jerry Marek, a California mortician and former coroner who examined Mr Russell’s body, said he had suffered a blow to his head and that his body did not have bruises and fractures consistent with a car accident.

Mrs Russell and the former Decai employee said Mr Russell was a dedicated teacher, but Ms Luo insisted he was often absent from class and that his “teaching methods failed to meet the requirement of the school and fit the students”. She said he had been hired on probation, which he failed partly because of a drinking problem.

“It was very strange and irresponsible for them to blame us for their son’s death,” Ms Luo said.

Mrs Russell denied that her son drank while teaching at Decai.

Established organisations that place foreign teachers report a growing number of less qualified people who want to teach English in China, who have not been vetted before arrival, and are not given training beforehand.

Voluntary Service Overseas, has long-term arrangements with local government and long-established schools, reported few problems for its 80 or so teachers. But Li Guozhi, VSO director for China, said people should not “just come along and look for a teaching job in China”.

Arriving without due preparation and qualifications would, he said be particularly risky in coastal areas where many private schools hire directly. For them, foreign teachers are a matter of prestige to attract rich parents to the school.

Mr Li said teachers need to be well-prepared and visit the school before signing a contract. “Private schools can demand that foreigners teach many hours a week,” said Mr Li. “They can cut the teacher’s salary if those hours are not met. As long as these schools operate as a business they will be concerned about money.”

For Ms Davis, coming to China meant a chance to see the world outside Ystradgynlais, her Welsh village. She said she loved her students, but the harsh conditions became too much for her.

At first, the school had said that Ms Davis and her boyfriend could forgo the last two months of their assignment, but the principal changed his mind the day before their departure and refused to be reasoned with, she said.

Repeated calls to the school went unanswered.

Ms Davis said that when they left the school that March morning, they took the cab to the station to take a train to Beijing, but they were so worried that they would be stopped and forced to stay that they decided to take the cab the whole 200 miles.

As they set off, their cab driver happened to be playing the theme tune from The Benny Hill Show on tape. “We just burst out laughing,” Ms Davis said.

The couple never collected their salaries for their last month of work.

A GROWING EDUCATION SYSTEM

* China has the world’s largest education system, with 367 million students under 18 years of age and 12.5 million teachers.

* There are 584,000 schools, of which 91,500 are secondaries and 491,000 are primaries. The rest are special and vocational schools.

* Teachers’ salaries range from Yuan 1,400-2,200 (pound;110-210 ) per month but foreign teachers would also expect to have housing provided.

* There are about 60,000 registered foreign English teachers, excluding those in part-time jobs and foreign students or expats teaching English alongside their studies or other jobs.

Source: China Bureau of Statistics (2004-05)