Bank One in the US saw its shares plunge by 25 per cent yesterday morning after a warning that the profitability and growth rate of its First USA unit, which claims to be the world's biggest issuer of Visa cards, was ebbing.

The shares had fallen by $13 (€12.41) to stand at $42 in early trading. Its difficulties may signal imminent change in the credit card sector.

That should be a relief for the average American credit card user. Two years ago, credit card companies were sending out direct mail offers for cards at a rate of around 2.25 billion a year: the rate has now jumped to as many as 3.5 billion. But the direct mail wars are starting to take their toll.

While Bank One shocked the stock market with its warning on Tuesday, the pressures that caused it to slip up are not new.

"The industry dynamics have been changing gradually over the last two years," said Mr Mike Urkowitz, head of Chase Manhattan's credit card business. Credit card lending has been slowing, despite the continuing spending binge. In late 1997, new lending volumes were running 10 per cent higher than a year before. That rate has fallen to around 2 per cent now. At the same time, competition between card issuers has become fiercer following mergers in the industry.