The Musicians' Union has launched a new campaign to promote live music in theatres after a West End show replaced players with recorded music last week.
Les Miserables has sparked the campaign
The Keep it Real! drive follows the replacement of 11 musicians in a production of Les Miserables with computer-generated music.
The campaign asks theatregoers to boycott performances where live music has been replaced by recordings.
The union also said tickets should be cheaper if music was recorded.
"For some time now the Musicians' Union has been aware of a growing trend in the use of recorded music to replace musicians and singers in live performance," said Horace Trunbridge, the union's assistant general secretary.
"From something as traditional as a pantomime, to the stadium tour of a chart topping pop act, backing tapes and miming are becoming increasingly prevalent," Mr Trunbridge said.
"The union believes that a live performance should be just that - live."
He said the savings producers made by replacing live music with electronic sinfonia were very rarely passed on to ticket buyers.
He said using backing tapes devalued the experience of seeing live performances.
"Audiences will enthuse about when they saw and heard The Beatles at the Cavern, Pavarotti in the Park, Bob Marley at the Lyceum, The Pistols at the 100 Club.
"However, would these legendary music events have been so memorable if they had all been performed using backing tapes? One performance would be no different to the next or the one before," Mr Trunbridge said.