diff --git a/var/spack/repos/builtin/packages/flex/package.py b/var/spack/repos/builtin/packages/flex/package.py
index c99c8ee4ae7..768227fa887 100644
--- a/var/spack/repos/builtin/packages/flex/package.py
+++ b/var/spack/repos/builtin/packages/flex/package.py
@@ -66,6 +66,10 @@ def flag_handler(self, name, flags):
                 iflags.append("-Wno-error=implicit-function-declaration")
         return (iflags, None, None)
 
+    def setup_build_environment(self, env):
+        if self.spec.satisfies("%oneapi@2023.0.0:"):
+            env.set("CFLAGS", "-Wno-error=implicit-function-declaration")
+
     @classmethod
     def determine_version(cls, exe):
         output = Executable(exe)("--version", output=str, error=str)