A locking device for preventing unauthorized retraction of a telescopic prop is made up of an adjustable clamp which can be fitted round the outer circumference of the prop together with a fastener for tightening the clamp, such as a nut and bolt with anti-tamper heads and a shield to prevent access to the head with a gripping instrument. The device can be placed around the outer circumference of an extended prop adjacent to the collar such that once the clamp is tightened the collar is sandwiched between the locking device and the pin. Alternatively the device can be placed around the outer circumference of the inner tube of the prop adjacent to and abutting the end of the outer tube of the prop in order to prevent unauthorized retraction of the prop.