Under which condition is a flex thrust takeoff prohibited?

A flex thrust takeoff is designed to optimize fuel efficiency and engine wear during normal operations, but certain conditions require maximum thrust for safety reasons. When wing and/or cowl anti-icing bleeds are in use, the engine's performance is affected due to the extra load from the anti-ice system. This can reduce the thrust available, meaning that using flex thrust, which relies on reduced thrust settings, could compromise engine power necessary for a safe takeoff, especially in cases when maximum thrust may be warranted – such as on a slippery runway or in specific weather conditions. Consequently, operating with anti-icing systems engaged necessitates a full thrust takeoff to ensure full engine performance and safety during departure.