Banks are battling to win credit card customers by offering some eye-catching deals on balance transfers.

Barclaycard's Platinum Credit Card with Extended Balance Transfer tops the table with the longest ever 0% period of 27 months, double the typical 0% period five years ago. Other card issuers are hot on the heels of Barclaycard. Royal Bank of Scotland (RBS) and Virgin Money have both recently extended their 0% balance transfer periods to 26 months. Or there's a deal from Halifax that charges zero interest for 25 months.

David Buxton, managing director of credit at Virgin Money, says: "Many customers really value the extra breathing space that an interest-free balance transfer period can provide."

Balance transfer cards are targeted at customers who have already built up a debt on another card or cards at a high rate of interest.

If you switch the outstanding balance to a 0% card, you can potentially save hundreds of pounds, as long as you pay off your debt within the zero-interest period. For example, a customer with a balance of £2000 paying interest of 16.9% could save £599 by switching to the Virgin Money Credit Card.

Watch out for fees, though. Most 0% balance transfer cards charge a fee, typically about 3% of the outstanding amount. So, if you transfer a balance of £3000, the fee could be £90.

However, customers can take advantage of recent falls in transfer fees. The average balance transfer fee is now 2.81%, the lowest level for five years, according to Moneysupermarket. The average fee last year was 3.04%.

In other words, if you switched a balance of £3000 last summer, you would have paid a fee of £91.20, compared to £84 now. Fees vary between providers and even between deals. Cards with the longest balance transfer offers usually charge the highest fees.

Most experts recommend that you set up a direct debit to make sure you clear the transferred balance before the 0% deal expires. Otherwise, you will start to rack up interest on your debt – and interest rates are rising.

Moneysupermarket figures show average rates currently stand at 18.7%, up from 16.44% in 2009. If you don't think you'll be able to clear your balance by the time the interest-free period ends, you should perhaps opt for a card with a lower interest rate, such as Sainsbury's Low Rate Nectar Credit Card at 7.8%.

Some 0% balance transfer cards also charge zero interest on purchases, though the interest free periods are rarely as long. Virgin Money, for example, offers zero interest on purchases for just six months.

But you can get a better deal if you look for a 0% purchase card. For example, Tesco Bank charges 0% on purchases for 16 months, or RBS, M&S and Halifax all offer deals at 0% for 15 months. If you plan to shop with a credit card, it is therefore often best to take out two separate cards to maximise your savings.

The top credit card offers aren't available to everyone. Credit card firms reserve the best deals for the best customers – and the best customers usually have a clean credit record.

So, if you have struggled with debts in the past, your application could be turned down. It's also worth bearing in mind that you might not be offered the advertised rate. The rules state that only 51% of successful applicants must pay the advertised rate, which means that almost half could be paying a much higher rate of interest.

In addition, the 0% cards typically exclude existing customers and you cannot normally transfer a balance from a card within the same banking group. For example, you could not switch a debt from a Bank of Scotland card to a Halifax deal as the two banks are part of the same group.

Andrew Hagger, of MoneyComms.co.uk, warns consumers of another potential pitfall – so-called personal pricing. "If you take a close look at the credit card summary box information you will find that some providers will offer a much shorter introductory 0% term to some customers. In a number of instances, the 0% term actually offered can be less than half of the headline deal shown in the best buy tables.

"A worst case scenario for someone who is accepted for the latest Barclaycard 27-month balance transfer deal is they end up with 12 months at 0% and an APR of 29.9% – an almost unrecognisable product compared with headline 27 months and 18.9% APR."

But only 31% of credit card customers are aware of personal pricing, new research from Consumer Intelligence shows.

Card companies have to give the advertised rate to at least 50% of successful applicants but that means the rest can be offered higher rates or shorter introductory rate periods. Many applications continue to be turned down entirely. Applications depend on an applicant's income, expenditure, credit rating and level of indebtedness, and can result in surprises for customers believing they are entitled to the advertised rate.

David Black, of Consumer Intelligence, said: "Consumers not surprisingly assume the advertised headline rate is the rate they will pay if they are accepted. In reality it is not that simple and the market has changed."