There is provided a method of operating a wind turbine which has at least two rotor blades. A first ice detection method is performed by a first ice accretion detection unit. A first warning signal is output if an ice accretion which exceeds a first threshold value is detected at one of the rotor blades by the first ice detection method. A second ice detection method is performed by a second ice accretion detection unit. A second warning signal is output if an ice accretion which exceeds a second threshold value is detected at one of the rotor blades by the second ice detection method. An enable signal is output if a freedom from ice is detected at the at least two rotor blades by the second ice detection method. Intervention in the operation of the wind turbine is effected by a control unit if the first or second warning signal has been detected. Intervention in the operation of the wind turbine is deactivated by the control unit if the control unit receives an enable signal and has previously received the second warning signal.