The Federation of Small Businesses (FSB) said credit card fees "charged by the handful of card providers which dominate the cashless payment space have soared in recent years".
"Small businesses are almost always charged more for card terminals than big corporates - so when online giants start throwing down the gauntlet, you know the situation is becoming critical," said FSB national chairman Mike Cherry.
Businesses have had ongoing concerns about credit card fees from the major providers.
Last October, Visa and Mastercard were
accused by the British Retail Consortium (BRC) of charging excessive fees. The trade body said they had doubled in two years.
At the time, the BRC warned that retailers would be forced to pass on costs to consumers, with credit card bills rising by up to £40.
The Payment Systems Regulator said there were "real questions about how well the cards market is working".
"We will look into how well this market is working, including the issue of increasing card fees," a spokesman for the regulator said. "If necessary we will intervene to address any issues we identify."