'Everyone expected us to be deported, but it didn't happen,' said the 45-year-old Canadian, referring to the three-month prison sentence handed out to him and fellow teacher Allan Parson.

The pair was nabbed in a police raid on a Bangkok school in January after a random background check of 1,000 foreign teachers revealed that 65 had used bogus university certificates.

The clampdown highlighted a problem that has been common for years in the teaching industry in Thailand, but is now gaining fresh attention.

Last year alone, 6,807 foreign nationals applied for a visa to teach in Thailand. Many of those applying for a five-year teacher's licence used counterfeit documents purchased on Bangkok's backpacker streets near Khao San Road - home to budget travel agencies and dodgy dealers of counterfeit documents.

Thailand's reputation for shoddy vetting procedures used by schools to employ foreign teachers is nothing new, with the issue being highlighted by last year's scandal surrounding US school-teacher John Mark Karr over alleged links with the murder in 1996 of six-year-old Colorado girl JonBenet Ramsey. Despite his outstanding warrant in the US for other charges, Mr Karr had been teaching in Thailand since 2005.

The subsequent publicity prompted the government to set up more stringent regulations for foreigners. Last October, the Immigration Police Bureau (IPB) began enforcing a measure limiting tourists to a 90-day limit of stay in the country on successive 'visas-on-arrival'.

Many teachers who lacked credentials needed for a one-year visa would leave the country every 30 days to get their passport extended another month, making fake credentials even more popular.

'It costs about 1,000 baht [HK$230] for a degree and another 1,000 for the transcript,' said one British teacher working in the countryside north of Bangkok using fake papers. 'The more you pay the better quality you get.

'You just scan it and send it to the agent. When it's scanned they can't really check the quality of it. The transcript on the back doesn't have any marks so you know it's a dodgy.'

The problem of fake credentials extends to the administration level, with many foreign instructors on Thailand's popular expat websites such as thaivisa.com claiming that many schools advise potential teachers where to get the fake diplomas.

Coincidentally, the recent crackdown and arrest of Mercer and Parson came days after the deportation of another foreign teacher wanted by British police for raping a minor in Britain in 1998.

Sean McMahon, 45, who was reportedly employed by an international school for two years in Bangkok, was swiftly repatriated within days of his arrest on January 15.

But amid greater vigilance by authorities, the future is uncertain for many foreign instructors.

Kevin Jesuino arrived in Thailand 16 months ago without a degree. He now teaches part-time English at a school outside the capital, but the brunt of his work involves organising special English camps for youngsters.

With an extensive background in dance instruction, the 25-year-old Portuguese-Canadian works well with the children, but the teaching guidelines being enforced by the school are still unclear.

'At our school it's becoming mandatory to have a degree. It wasn't mandatory when I arrived more than a year ago. There's no standard procedure really. Every province has its own process.'

Earlier this year the International School Association of Thailand voiced concern over the new lengthy process enforced by the immigration police, including strict criminal background checks of applicants.

Prisoners' rights advocate Debbie Singh said the jail terms meted out amid the recent crackdown were too harsh and cited the dangers foreigners faced in Thai jails. 'Prisoners sleep on the concrete floor sometimes up to 40 in a room. It's like a can of sardines. If you wake up in the night and want to go to the toilet, a hole in the corner of the room, you are stepping over bodies,' Ms Singh said.

'You got the diseases - TB, diarrhoea - the hygiene factor just isn't there.'

But the IPB's chief Lieutenant-General Suwat Tumrongsiskul has no misgivings about the tougher regulations and penalties.

'The Canadian got 90 days to make an example - to show foreign teachers using fake documents that we are serious,' he said. 'After the John Mark Karr publicity we are more concerned about the teachers and about protecting our students.

'We want to make it convenient for foreign teachers who want to teach legally.'

But their time in jail will not be a total waste for Mercer and Parsons as prison authorities have given them jobs.

'They've got us teaching English to the inmates,' Mercer said.