The Royal Academy of Music, the Guildhall school of music and drama, and Birmingham Conservatoire are concerned about the lack of interest.
Curtis Price, principal of the Royal Academy, said: "Some of it is to do with the closure of the pits. There is no longer such a strong connection with the tradition of brass bands. With fewer children taking up the instruments, generations of talent will be missed."
Peter Gane, Guildhall's head of wind, brass and percussion, said he had noted "a gradual creeping drop", and Richard Shrewsbury, the Conservatoire's education and recruitment manager, said in the past decade application numbers for instruments such as the euphonium and trombone had fallen.
The comments came as the Department for Education and Skills completes a survey on pupils' uptake of musical instruments, due this autumn.
Wes Lawrence, music service manager for North East Lincolnshire, which runs a project to promote lower brass instruments, said: "If you don't have kids playing these instruments in orchestras, it threatens the others who want to play in ensembles."