Ticket agencies are adding up to 28% to the face value of a theatre ticket, according to a Which? report.
Bombay Dreams is one of the most successful West End shows
Some are also flouting industry rules by failing to tell callers about extra charges and small print, it says.
The report found four tickets to a West End show could differ from £108.45 to £127.40 depending on the agency used.
"Paying a 28% fee is equivalent to a family of four taking one of the agency staff along for free," said Which? editor Helen Parker.
The Society of Ticket Agents and Retailers (Star) was formed by the ticket industry to promote high standards of service.
One of its main guidelines is that members should "clearly identify the face value of any tickets purchased and any additional booking fee" as well as any terms and conditions which may apply to sales.
But Which? researchers posing as customers rang five ticket agencies four times to buy tickets for Bombay Dreams and We Will Rock You, finding some flouting the rules.
Star members Utickets and Abbey Box Office failed to make the ticket value or fees clear in any calls, and did not give a breakdown of charges even when asked, said the report.
Firstcalltickets did not explain its terms and conditions and gave the ticket value in only half the calls.
Researchers also contacted Wayahead, which is not a Star member, and found it failed to make clear its charges.
Although it is not breaking any laws, there is concern that they are breaking Star regulations.
Only Ticketmaster, one of the world's largest agencies, passed the test with flying colours
Abbey Box Office customer services manager said that although it is a smaller agent it is prides itself on its service delivery and that its fees are not unusual.
"Possibly the reason the clerk did not make the charges explicit is that the majority of our clients are repeat customers who know our terms and conditions and come back to us to buy again and again, or maybe the clerk made a mistake, as occasionally everyone makes a mistake," the company said.
Jonathan Brown, secretary of Star, said although there were differences in booking fees and charges, using one of its members was the only way to ensure customers had a recourse in case of a complaint.
He added that in light of the Which? report several of its members were implementing training to ensure all staff were familiar with the guidelines.
The Society of London Theatre (SOLT), which represents theatres and producers, said its advice would be to go direct to the theatre box offices to buy tickets, or to shop around through Star members.
A spokesman also said that a number of years ago it had attempted to get agencies to agree uniform fees and prices but had been threatened by the Office of Fair Trading that this would be unlawful.