A failsafe valve actuator, the valve actuator comprising: a drive mechanism; a drive shaft; the drive shaft comprising an externally threaded portion extending along at least a portion of the length of the drive shaft, and being coupled to the drive mechanism, such that the drive shaft is rotatable by the drive mechanism when the drive mechanism is energised; and, wherein the drive shaft is further axially moveable between a retracted position and an extended position; a drive shaft bias member arranged to axially bias the drive shaft into a failsafe position; a clutch concentrically arranged around the externally threaded portion of the drive shaft; the clutch comprising an internally threaded portion on an inner surface of the clutch complementary to the externally threaded portion of the drive shaft; and, a clutch actuator coupled to the clutch and arranged to move the clutch between an engaged position in which the internally threaded portion of the clutch engages with the externally threaded portion of the drive shaft and a disengaged position in which the clutch is disengaged from the drive shaft; wherein in response to energisation of the clutch actuator the clutch is moved to the engaged position such that threaded engagement between the drive shaft and the clutch transfers rotational movement of the drive shaft by the drive mechanism into axial movement of the drive shaft and holds the drive shaft against the drive shaft bias member when the drive mechanism is inactive; wherein movement of the clutch to the disengaged position, causes the drive shaft bias member to automatically return the drive shaft to the failsafe position; and, characterised in that the clutch comprises first and second shells (22, 24) with a semi-circular shaped cross-section; the first shell comprising a smooth inner surface proximal to the drive shaft, and the second shell comprising the internally threaded portion; and, wherein an external diameter of the second shell is smaller than an external diameter of the first shell. a drive mechanism; a drive shaft; the drive shaft comprising an externally threaded portion extending along at least a portion of the length of the drive shaft, and being coupled to the drive mechanism, such that the drive shaft is rotatable by the drive mechanism when the drive mechanism is energised; and, wherein the drive shaft is further axially moveable between a retracted position and an extended position; a drive shaft bias member arranged to axially bias the drive shaft into a failsafe position; a clutch concentrically arranged around the externally threaded portion of the drive shaft; the clutch comprising an internally threaded portion on an inner surface of the clutch complementary to the externally threaded portion of the drive shaft; and, a clutch actuator coupled to the clutch and arranged to move the clutch between an engaged position in which the internally threaded portion of the clutch engages with the externally threaded portion of the drive shaft and a disengaged position in which the clutch is disengaged from the drive shaft; wherein in response to energisation of the clutch actuator the clutch is moved to the engaged position such that threaded engagement between the drive shaft and the clutch transfers rotational movement of the drive shaft by the drive mechanism into axial movement of the drive shaft and holds the drive shaft against the drive shaft bias member when the drive mechanism is inactive; wherein movement of the clutch to the disengaged position, causes the drive shaft bias member to automatically return the drive shaft to the failsafe position; and, characterised in that the clutch comprises first and second shells (22, 24) with a semi-circular shaped cross-section; the first shell comprising a smooth inner surface proximal to the drive shaft, and the second shell comprising the internally threaded portion; and, wherein an external diameter of the second shell is smaller than an external diameter of the first shell.