Aito has written to the Office of Fair Trading after it emerged that high transaction fees for customers using ‘premium’ credit cards were costing its membership £600,000 a year.

While the normal agreed transaction fee is 1.5%-2%, operators said the charge for premium cards – those that offer extras such as free travel insurance or cashback – was at least 3%.

Aito investigated the matter after members said they were receiving higher bills than usual from the banks’ merchant acquirers.The premium cards now account for one in five payments made to members.

Aito chairman Derek Moore said: “Holiday companies are not high-margin, high-profit businesses.

“Many members will make as little as 2% to 5% on the sale of their carefully-crafted holidays.

“To find that such modest margins are hit by unexpected and unbudgeted additional charges is a big shock and we believe it is totally unreasonable.

“All charges of this type must be transparent, by which we mean that they must be specified clearly and in advance.”

Aito is calling for urgent legislation to be brought in to stop banks being able to impose such charges without notice. In the meantime, operators said they may have to increase the levy on all credit card users.

Noel Josephides, managing director of Sunvil, said: “This is coming at a time when companies are being criticised for hidden charges for customers, and yet we are being subjected to exactly the same treatment by the banks.

“There is no way of knowing which these cards are, so the tour operator doesn’t know what he’s going to be charged. The government should be taking action against the banks.”