According to a statement on the ASA website: "We found that compulsory fees and charges were not being included up-front in ticket prices. This meant that fees charged on either a per ticket or per booking basis were only made clear later in the buying process."
The four venues and agencies ruled against are AKA Group, The Old Vic, Charing Cross Theatre and Ambassador Theatre Group.
Guy Parker, chief executive of the ASA, said: "These pricing practices are simply not fair. They draw us in on a false promise. They make us less likely to shop around for the best deal. Our rulings send a clear signal to advertisers: sort out your pricing so we all get a fair deal."
The Charing Cross Theatre - currently staging the British premiere of Jerry Herman's Dear World - has agreed as a result to mention booking fees on the first page of the booking process, though the venue's producer Steven Levy expressed doubts over the fairness of the ruling.
Speaking to Whatsonstage.com, Levy said: "I think the ASA is addressing a non issue because I would argue that virtually every theatregoer knows that in order to buy tickets online, most venues charge a booking fee. The bigger question is why a company such as TheTrainLine.com adds its fee on the very last page of the booking. Why is the ASA not going after people who are wilfully trying to withold this information until the very last minute?"
A statement from the Ambassador Theatre Group said: "The ASA advised us that informing customers at the beginning of the booking process that a fee or charge may apply and to then confirm the amount of that fee later in the booking process needed revision. We have embarked on a series of improvements to the information on our website to ensure that ticket prices and any fees charged for purchasing online are clearly available to customers at the beginning of the booking process as well."
A spokesperson from AKA Group told the BBC: "Moving forward we will ensure that the first references to pricing for a performance are immediately qualified with the amount of any associated transaction fee or charges."
The Old Vic, which was censured for neglecting to mention booking fees during the seat selection process for Kiss Me, Kate, said it had now added text to the bottom of the page which highlights the amount of the booking fees.