A speed reducer, in particular for torque transmission between a gas turbine and a fan in a turbomachine, including an annular ring gear coaxial to a main axis of the speed reducer and having radially inner teeth, at least one gear which is coupled with the ring gear and which includes radially outer teeth complementary to the teeth of the annular ring gear, wherein the ring gear includes two annular half-ring gears offset with respect to each other along the main axis of the speed reducer and which are coupled with said at least one gear, and a support plate for supporting both half-ring gears, with respect to which both half-ring gears are rotatably immovable about the main axis of the speed reducer, each half-ring gear including helical inner teeth tilted in a direction opposite to the tilt direction of the teeth of the other half-ring gear, wherein each half-ring gear is connected to the support plate so as to be able to be uncoupled from said at least one gear when said at least one gear exerts on each half-ring gear an unclutching action the amplitude of which is higher than a determined amplitude value, characterised in that each half-ring gear is slidably mounted with respect to the support plate, along a direction parallel to the main axis of the speed reducer, between a coupled position in which said half-ring gear is coupled with said at least one gear and an uncoupled position in which said half-ring gear is uncoupled from said at least one gear, and in that the speed reducer includes retaining means (48, 60) for retaining said half-ring gear in its coupled position, said retaining means (48, 60) being able to be disengaged to enable said half-ring gear to be moved to the uncoupled position when said at least one gear exerts on each half-ring gear said unclutching action. an annular ring gear coaxial to a main axis of the speed reducer and having radially inner teeth, at least one gear which is coupled with the ring gear and which includes radially outer teeth complementary to the teeth of the annular ring gear, wherein the ring gear includes two annular half-ring gears offset with respect to each other along the main axis of the speed reducer and which are coupled with said at least one gear, and a support plate for supporting both half-ring gears, with respect to which both half-ring gears are rotatably immovable about the main axis of the speed reducer, each half-ring gear including helical inner teeth tilted in a direction opposite to the tilt direction of the teeth of the other half-ring gear, wherein each half-ring gear is connected to the support plate so as to be able to be uncoupled from said at least one gear when said at least one gear exerts on each half-ring gear an unclutching action the amplitude of which is higher than a determined amplitude value, characterised in that each half-ring gear is slidably mounted with respect to the support plate, along a direction parallel to the main axis of the speed reducer, between a coupled position in which said half-ring gear is coupled with said at least one gear and an uncoupled position in which said half-ring gear is uncoupled from said at least one gear, and in that the speed reducer includes retaining means (48, 60) for retaining said half-ring gear in its coupled position, said retaining means (48, 60) being able to be disengaged to enable said half-ring gear to be moved to the uncoupled position when said at least one gear exerts on each half-ring gear said unclutching action.