Joanne Nichols spent months trying to find a job after she was laid off in 1992. Then she spotted an advertisement in her local paper for driving instructors.

First she attended a seminar. 'They said the earning potential was about pounds 20,000 a year and we'd be able to start teaching within six months - after a pounds 1,500 course.'

Ms Nichols, who is married with three children, borrowed the money via a government career development loan, which allowed her a six-month repayment 'holiday'. The company would be owed a 'franchise fee' for the car and the use of its logo, but if you could earn 20 grand . . . no worries.

Fifteen months later she still has not passed the Driving Standards Agency (DSA) exams. She has had to start loan repayments and is doing whatever temporary work she can find. It looks as if it will take her three years to clear the pounds 1,500 debt - if she can hold on to a job.

And what does this mean for her prospects of becoming a driving instructor? 'There is no prospect now. I'm working 39 hours a week and I've no time left to train.'

The news last month that 100 L-test centres are shutting down because fewer people are taking tests will come as the final straw for thousands of people around the country who, like Ms Nichols, have become enmeshed in the driving instruction business. Many are now broke or deeply in debt.

The figures proffered by the DSA to justify its closure plans have been known within the business for ages. Those same figures, however, are seldom bandied about by driving schools that teach instructors. It is hardly in their interests to warn new recruits that Britain already has 33,000 qualified driving instructors, or that an estimated one third of them are out of work for lack of pupils.

Yet every year 20,000 hopeful, sometimes desperate, men and women sign up and sink their redundancy money into becoming Approved Driving Instructors.

Bob Williams, an instructor of many years' standing, whose book on the subject is recommended reading for would-be instructors, complains: 'So many people think they can get into the business simply because they can drive. They don't understand how hard it is to pass and how high the failure rate is. After all, none of the schools is going to emphasise that, are they?'

There are three parts to the test. The first is a theory exam. For every 1,000 people who take it only 600 will pass. The second hurdle is an advanced driving test. Of those 600 only 300 will get through.

The last and most difficult part is the test of ability to instruct. This ensures that the instructor can correct a pupil without destroying confidence, and can be calm under pressure. Of the 300 still trying, only 100 will pass.

Finally, of the 100 who are now fully qualified to teach, only 40 will pass the first spot check made by DSA inspectors. And there is not even enough work for them.

'A pass rate of 4 per cent is ridiculous,' says Peter Russell, general secretary of the the Driving Instructors Association (DIA). 'It borders on the criminal. And for every 3,000 driving instructors who qualify every year, 2,000 go out of business within 12 months.'

What about the claims of an earning potential of pounds 20,000 a year or more? The DIA says this is unlikely for someone just starting out. You normally have to work at least 20 hours just to pay back a franchise fee to your company. At one top school that will come to about pounds 300 a week. Fuel, insurance and other costs come on top of that.

Mr Williams agrees: 'It's really only the top instructors who are earning this sort of money, and with so many qualified there's lots of price-cutting, so it's even harder to start off at the top earnings.'

The driving school that recruited Ms Nichols is Intercounty, based at Bromyard, Herefordshire. It proudly claims to turn out 18 new instructors every month. The managing director, Alan Dorrofield, says the company vets its candidates closely and believes it is right to encourage people to borrow money to train rather than be thwarted. He also says that everyone who sticks at the course passes.

A 100 per cent pass rate? 'Can't guarantee 100 per cent. Some have dropped out, but very small numbers - just one or two - in all the hundreds we deal with.'

Sue Wade is also from Grantham, also jobless and encouraged by the thought of a new career. She too, took a career development loan and has had to repay it without qualifying. 'I can't afford to carry on,' she says. 'In any case, now I've researched the market. There isn't likely to be anyone there for me to teach if I succeed.'

I went to a part one exam in Birmingham and conducted a straw poll to find out how many of the candidates were aware of the facts about what they were going into. Many of them were hoping to salvage a new life out of the chaos of redundancy, many had borrowed money, and most thought it would take them only six months or so to complete the course.

Asked why they were convinced there was a market for driving instructors, one man said: 'You only have to look at the volume of traffic on the roads.' Another said he hadn't thought about it and one woman, challenged on her assertion that 'more and more people want to learn to drive', retorted: 'It comes down to advertising, and if you're good you'll make it.'

I agree that it really is down to advertising . . . and it is the driving schools who are good at that.

The author presents BBC 2's 'Top Gear'.