Tens of thousands of credit card holders face a sudden increase in interest rates in the run-up to Christmas before the introduction of a new industry standards agreement, struck yesterday, which should offer better protection to borrowers next year.

Britain's biggest credit and store card companies have signed up to a new set of principles to ensure that 30.8 million customers are treated fairly. From January 1, cardholders will be given at least 30 days' warning of interest rate increases.

Credit card companies will also be banned from raising rates in the first year that a card is issued and will be able to review interest rates only on a six-monthly basis.

Apacs, the UK payment association, estimates that more than 10,000 cardholders have the interest rates on their credit cards increased every month.

Stephen Sklaroff, of the Finance & Leasing Association, the trade body, admitted that in a small number of cases the decision to increase interest rates will “have already been taken and would already be in the system”.

Malcolm Hurlston, of the Consumer Credit Counselling Service, a debt charity, said that it is was “regrettable but inevitable” that some borrowers will suffer interest rate increases before January 1.

The new agreement follows a credit card summit last month, which brought together credit card providers, debt charities and government ministers.

The Government accused the card companies of increasing interest rates by up to 10 per cent with little or no warning. It is also unhappy that interest rates have continued to rise as the Bank of England base rate falls.

Credit card companies argue that interest rates are based on the risk posed by borrowers, which is subject to change. However, the new rules will give borrowers more warning that their rate is about to increase.

Sandra Quinn, of Apacs, said: “Quite a few companies haven't done any risk-based repricing this year, which suggests that the real gain with these principles will be next year when arguably more risk-based repricing would have been occurring.”

The new principles have been welcomed by debt charities for alleviating the pressure on over-indebted borrowers.

Credit card companies will freeze interest rates for borrowers who have sought help from debt advisers and will give customers the option of converting their debt into a personal loan with an interest rate that is equal or below the rate on their credit card.