An apparatus for sealing a pipe having a defect or penetration comprises a housing coupled to the pipe by a clamp member. A piston is mounted in the housing and the piston and housing define a chamber. The piston further comprises a port for providing fluid communication between the pipe and the chamber. A seal member is provided between the piston and the pipe and, in use, the apparatus is located over and/or around the defect or penetration in the pipe such that the seal member engages the pipe. The piston is urged towards the pipe by a fluid pressure force generated by fluid in the pipe thereby maintain sealing engagement between the seal member and the pipe and to prevent leakage of fluid through the penetration.