If your credit card balance runs into thousands you may be tempted to switch to one of the growing number of rival cards promising rock- bottom interest rates. Many borrowers are paying upwards of 20 per cent on cash purchases on credit cards. Yet a number of recent card issues have set rates at only a fraction of this level.

In June, Abbey National's online bank offshoot Cahoot launched with a credit card charge of a (literally) unbeatable zero per cent, an offer that has now closed. But this week RBS Advanta, a subsidiary of the Royal Bank of Scotland, cut introductory rates to a market- leading 2.9 per cent APR on its Classic, Gold and Platinum cards.

Other competitive introductory deals are available from building society Nationwide, which offers 3.5 per cent, while Capital One Bank offers 3.9 per cent and Marbles charges 4.5 per cent. Most of these deals apply both to new purchases and the balance outstanding on your existing card, if you transfer it to your new issuer. So if you are paying, say, 19.9 per cent on Barclaycard, it could be time to switch.

The introductory rates last for a set period, typically six months, although RBS Advanta lasts slightly longer, until June 2001. Thereafter, you revert to the card issuer's standard rate. Check this rate carefully, as this is what you will be paying in the longer run.

RBS Advanta's introductory rate shoots up to 17.9 per cent (Platinum 16.9 per cent), Nationwide increases to 14.6 per cent, Capital One Bank to 12.9 per cent and Marbles to 14.9 per cent. All still better than Barclaycard. But then, why pay this rate at all? Canny readers will have realised it is possible to enjoy a life of low-cost credit card borrowing, by hopping from one issuer to another when the introductory rate is over.

You may assume there is a catch to card surfing, after all lenders are not famed for their generosity, but there isn't. Few credit cards have the type of lock-in period imposed by many attractive mortgage deals. You are free to switch at any time.

"There are no lock-ins on any of these credit card deals, so people can switch from one low rate to another in theory," admits Mark Austin, marketing manager for Tesco Personal Finance.