A Catholic school has cancelled a speaking visit from an award-winning gay author of young adult books because his new book - which includes a gay character - has been deemed "not appropriate".

Will Kostakis, 26, came out as gay on his blog last week after an ex-boyfriend of his was diagnosed with cancer.

Young adult author Will Kostakis. Credit:Nic Walker

Just days later he received a cancellation email from a teacher at De La Salle College in Revesby Heights, in Sydney, where he had done a successful speaking visit last year.

"We have a concern about promoting your new book at our school as it is a Catholic school," the email reads.