The report by the consumer campaigning organisation alleged 11 out of 24 hotel chains did not initially display VAT in the headline price of the cost of staying in their venues.
The investigation came about after the Advertising Standards Agency (ASA) upheld a complaint against InterContinental Hotels Group (IHG) in May for the UK-based group's use of VAT-exclusive prices in online adverts for its Crowne Plaza hotels.
The Which? Travel report accused brands including Doubletree by Hilton, Marriott, Holiday Inn and Accor's Mercure and Sofitel of displaying an initial price in the booking process that did not include VAT - an action that is thought to be in breach of trading standards and ASA guidelines.
However Thomas Dubaere, managing director of Accor UK & Ireland, said the global operator was committed to offering 'transparent pricing'.
"The majority of our 185 hotels, including all our Mercure hotels, display VAT inclusive prices throughout the entire booking process. There are currently four hotels that show VAT on the second page and by September 17, VAT inclusive prices will be displayed throughout the entire booking process."
When the ASA upheld a complaint against IHG earlier this year the authority said the operator had indicated a willingness to display VAT-inclusive prices and direct its franchisees to do the same.
However IHG had said it faced technical challenges in doing so and would only make the change if other hotel groups were also compelled to do the same.
In its report, Which? Travel cited Martin Couchman, deputy chief executive of the British Hospitality Association (BHA), who told the organisation many hotel chains were already planning to change practices as a result of the ASA action.
Hilton Worldwide is thought to be among operators planning on making the change to its UK websites with the ASA expecting the advertising to be changed by hotel chains by the end of next month.