The disclosure relates to a tube coupling body having a throughway open at one end to receive a tube and an insert sleeve to fit in the throughway adjacent the open end with one end of the sleeve located within the throughway and the other end at the end open of the throughway. The insert sleeve has a tapered internal cam surface reducing towards said other end of the sleeve with which fingers of a tube gripping collet are engageable to receive and lock a tube inserted into the throughway in the coupling body. The external surface of the sleeve has an annular groove with a metal grab ring embedded in the sleeve adjacent the side of the groove nearest said one end of the sleeve and having multiple teeth around its outer periphery projecting radially beyond the outer surface of the sleeve on insertion of the sleeve in the throughway, the resilient teeth are angled towards said other end of the sleeve by engagement with the surface of the throughway, the angling of the fingers being accommodated by the annular groove and the resilience of the teeth causes the teeth to grip with the surface of the throughway to prevent the insert sleeve from being withdrawn from the throughway.