Once the preserve of the rich and well-connected whose parents could arrange a trek with Masai or a job in Mother Teresa's Calcutta soup kitchen, gap years are now a must for any self-respecting student. They offer a chance to see the world, grow up and have colourful tales to tell during freshers' week.

Despite tuition fees and student debt, almost a third, or 130,000 sixth-formers, set out from the UK each year on a gap-year adventure before heading off to university, according to Mintel, the market analyst.

Few who take a year out ever regret it and most say they have made far more of their time at university as a result. But experts urge young people to plan their trips carefully and be aware of the pitfalls before they embark on the journey of a lifetime.

Gap years are now big business with students spending an average £4,800. Dozens of firms have sprung up to get their slice of the money. At the last count there were 76 operators in Britain and one gap-year company, i-to-i, was so successful it was bought for several million last year by the package holiday group First Choice.

These firms typically offer package trips combining flights, in-country transport, support and even language courses at an average cost of £4,000. However, it is their foray into charitable work that has provoked controversy. Students are increasingly eager to get off the backpacker trail and see real life, so they want to take on some voluntary work during their travels.

The Year Out Group, a trade association made up of 35 different gap-year travel companies, says that 80 per cent of young people now opt for a trip with a volunteering element.

But the big overseas volunteering charities are furious that these companies are charging students to volunteer. The charities also say there is little evidence that the thousands of students, who have few skills and little experience, are benefiting local communities in Africa, Asia and Latin America when they swing through for a month or so.

Voluntary Service Overseas (VSO), one of the biggest charities, has even taken the unusual step of urging young people to lie on the beach rather than get caught up in “voluntourism”. The companies have defended themselves, saying that not everyone has a year or more to devote to voluntary work but still want to make a contribution to the local community where they are travelling.

Companies that sell gap-year package deals with a stint as a volunteer will soon face a new code of conduct. Tourism Concern, a development charity, has begun work on a scheme that will give a quality mark to companies that offer up sustainable, long-term projects to customers.

Judith Brodie, director of VSO, warned students to ask plenty of awkward questions when signing up with a commercial company. “With the growth of the gap-year market, it is worth taking the time to do your homework before embarking on any trip. By asking the right questions you can ensure that the gap-year providers won't be the only ones who benefit from those taking time out.”

She advises checking on whether the project already exists and has not just been created to satisfy customer demand. “If you are keen on making a sustainable difference, it is worth finding out how many people had been on the scheme before you, how successful the project has been and how the work is set to continue after you leave.

“Effective development projects work in partnership with the local community. Be clear on how your project works with the local community, and how the work you do will have a lasting impact.”

Brodie also urges students to ask how the cost of their trip breaks down. “A good organisation will be honest and upfront about where your money is going and why, as well as how much your flights and accommodation will be subsidised, if at all.”

Case Studies: Gap years before and after university

Lauren Cohen was forced to take a gap year when she resat her A levels to get the grades she needed to study at St George's medical school at the University of London.

She chose a trip to Peru which combined voluntary work in a children's home in Lima with travelling and the Inca Trail. “I was very pleased with the company. It was well organised. The placement in the children's home was great and they found me accommodation in an old house in Lima which used to be the South American Explorers' club.

“Before I went I had a weekend induction course where I learnt all about Lima and Peru, got good health and safety advice on medication and what food to avoid. I also learnt a lot specific to the job.

“I paid extra to have a guide, which I think was worth it. It meant I was met at the airport and I got good advice on which areas to avoid in Lima. There are quite a few dangerous places. Maybe if I had been a bit older and a bit wiser I wouldn't have needed a guide but I was 19 and I didn't have a clue.”

She describes the experience as amazing. “I went to university with a totally different outlook. I really appreciated what I had far more as a result.”

Hannah Tetlow took her gap year after finishing her psychology degree at Oxford and before embarking on a graduate medical degree.

She thought the packages offered by gap-year companies were overpriced and eventually found a charity - Forever Angels in Tanzania - that she could volunteer for directly. “It was an orphanage set up by a teacher from England for babies and very young children.

“I suppose for me it had a lot to do with age. I was not 18 and fresh out of school. I was a bit older and had had some experience. Still, it was pretty petrifying arriving there and settling in and finding my feet. It helped greatly that Amy [Hathaway, the founder] is English. If you are not sure, or are anxious about travelling, maybe big agencies have a role. But if you can find a good charity it means that your money does not end up on a marketing budget. The money I brought out for a donation went straight to the charity.”

She says the experience was wonderful, if sometimes harrowing. “It was very upsetting to see the state these children were in. They had often just been left at the roadside and were malnourished and sick. A few months later they would be chubby and laughing.”

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