Method of regulating a speed, referred to as the NR speed, of driving in rotation at least one rotor of a rotorcraft, the NR speed varying in a predefined range of variation of NR speed under control of at least one control unit generating a setpoint, referred to as the NR setpoint, starting from which NR setpoint is regulated, by a regulating unit, the operating regimen of at least one turboengine supplying the mechanical power at least necessary for driving said at least one rotor at an NR speed in accordance with the NR setpoint, characterised in that the method comprises the following operations: -) detection of a movement of the rotorcraft under icing conditions, -) measurement by the on-board instrumentation of the rotorcraft of the ambient outside air temperature surrounding the rotorcraft and detection of a movement of the rotorcraft in an outside environment of an ambient temperature comprised in a predefined range of values, referred to as the critical temperature domain (Dct), identified between a predetermined high temperature (Th) and a predetermined low temperature (Tb), and then -) as a result of a detection of a movement of the rotorcraft in an outside environment of an ambient temperature comprised in said critical temperature domain (Dct), and taking into consideration respectively low (Ptb) and high (Pth) icing temperature ranges extending on one side and on the other side of a temperature, referred to as the median (Tm), of the critical temperature domain (Dct) comprised between said respectively low (Tb) and high (Th) temperatures, application by the control unit of a law for calculation of an NR setpoint, referred to as the law for NR calculation under icing conditions (Lg), according to the following modalities: - decreasing the NR speed in the case where the ambient outside air temperature is comprised in the low temperature icing range (Ptb) of the critical temperature domain (Dct), and - increasing the NR speed in the case where the ambient outside air temperature is comprised in the high temperature icing range (Pth) of the critical temperature domain (Dct). -) detection of a movement of the rotorcraft under icing conditions, -) measurement by the on-board instrumentation of the rotorcraft of the ambient outside air temperature surrounding the rotorcraft and detection of a movement of the rotorcraft in an outside environment of an ambient temperature comprised in a predefined range of values, referred to as the critical temperature domain (Dct), identified between a predetermined high temperature (Th) and a predetermined low temperature (Tb), and then -) as a result of a detection of a movement of the rotorcraft in an outside environment of an ambient temperature comprised in said critical temperature domain (Dct), and taking into consideration respectively low (Ptb) and high (Pth) icing temperature ranges extending on one side and on the other side of a temperature, referred to as the median (Tm), of the critical temperature domain (Dct) comprised between said respectively low (Tb) and high (Th) temperatures, application by the control unit of a law for calculation of an NR setpoint, referred to as the law for NR calculation under icing conditions (Lg), according to the following modalities: - decreasing the NR speed in the case where the ambient outside air temperature is comprised in the low temperature icing range (Ptb) of the critical temperature domain (Dct), and - increasing the NR speed in the case where the ambient outside air temperature is comprised in the high temperature icing range (Pth) of the critical temperature domain (Dct). - decreasing the NR speed in the case where the ambient outside air temperature is comprised in the low temperature icing range (Ptb) of the critical temperature domain (Dct), and - increasing the NR speed in the case where the ambient outside air temperature is comprised in the high temperature icing range (Pth) of the critical temperature domain (Dct).