The group has published a list of saints and sinners who it says implement a huge range of surcharges when consumers pay by credit and debit cards, with some hitting shopkeepers' pockets hard.
Which? previously ran a successful campaign pressuring the Government to ban rip-off card payment surcharges and the group has been monitoring how well the new rules are working.
Which? asked consumers if they felt they had been hit with a rip-off surcharge since the ban came into effect in April.
More than 700 people responded and researchers found that some companies were still charging excessive fees, with the worst offenders in the travel and holiday sector.
Which? said it believed several companies could be breaking the new rules.
It found eDreams was the worst offender and found examples of surcharges of more than 18% for customers paying by credit and even debit cards.
It also found easyBus, Vueling, Monarch, Jet2 and Germanwings were still charging over the odds with fees ranging from 2.5% to 3.5%.
Which? believes that this should be no more than 2% for credit cards and a matter of pence for debit cards.
Following complaints from consumers, Which? also contacted First Choice, Thomson, easyJet and Mytrainticket which all subsequently agreed to reduce their credit card fees to 2.0%.
EasyCar has gone even further and abolished all its fees for credit and debit cards.
Which? executive director Richard Lloyd said: " It's disappointing that six months after the Government banned rip-off surcharges, consumers are still being hit with high fees simply for paying with a card.
"While some companies have reduced their surcharges, there should be a crackdown on rogue companies who continue to flout the ban. We'll be passing on our findings to Trading Standards and asking them to enforce the rules."
Consumer Minister Jo Swinson said: "We know it can often be a nasty surprise when consumers see an advertised price and then have an excessive payment surcharge added at the end. This is why we brought in measures - a year earlier than required by European law - to make sure that UK consumers benefit from clearer and fairer prices.
"The prices that businesses charge should always be transparent. They shouldn't keep extracting more money from excessive administration or booking charges from customers with each transaction. Enforcers such as Trading Standards, the Office of Fair Trading and the Civil Aviation Authority have the power to seek civil injunctions against traders who break the rules on excessive payment surcharges. I will be writing to all the enforcement agencies about their obligations."
Sir Stelios Haji-Ioannou, chairman of easyBus, said the report was " misguided" and its average fare was about £6 with early bookers paying as little as £2.
He said: " Uniquely amongst our competitors, some 90-95% of our bookings are made online in advance and paid by credit/debit card.
"Our competitors still take most of their revenues cash in hand to the driver. No wonder they don't focus on credit card fees.
"EasyBus currently charges 50p per debit/credit card transaction compared with an approximate cost of 35p. It is astonishing that a 15p margin can be regarded as a 'rip-off' when easyBus undercuts alternative providers by half. This is a storm in a teacup over 15p.
"Which? clearly needs to rethink its raison d'etre."