Performers' union Equity says it has received "credible information" that children were racially discriminated against in the casting for a West End show.
It did not name the production but it is widely understood to be Mary Poppins at the King Edward Theatre.
Theatre group Delfont Mackintosh has been approached for comment.
Equity says it is investigating.
Youth casting agent Laura Walker said she previously spoke out against the show's casting director but later retracted her statement.
"I deleted my original tweet because I was worried I would be blacklisted and that my children would miss out on future opportunities. But things need to change."