Warwick Castle has been ordered to change the way in which card usage fees are shown to customers purchasing tickets to the attraction.
The Advertising Standards Authority (ASA) upheld a customer complaint that the website was misleading and the additional charges were not clear.
Online bookings incur a £1 charge for debit cards and Master Card transactions, with a £2 charge applying to Visa credit cards.
The ASA also backed up the customer’s complaint that the castle’s website was misleading because it displays prices exclusive of value added tax (VAT).
The castle’s operators, Merlin Entertainments, use similar charges and pricing layouts at its theme parks, which include Alton Towers and Thorpe Park.
The ASA ruling is likely to have a knock-on effect at all of the attractions operated by Merlin.
The British company has reportedly told the ASA that they are reviewing how their prices are displayed at all of their attractions.
Merlin is currently lobbying the British government to reduce the rate of VAT applied on hotels and tourist attractions from 20% to 5% – claiming it would create jobs and boost the economy.