A novel inflatable plug for interrupting fluid flow in a pipe is provided which effects a stable, rigid seal while at the same time has a minimal axial length so as to be easily maneuverable. The plug is made from flexible, impermeable fabric and is provided with finely distributed, flexible inelastic connector elements or threads running in the axial direction and secured to opposed walls. When the plug is inflated, the threads prevent the opposed walls from separating a greater distance than the length of the threads. A complete plug assembly is also disclosed, comprising three such plugs in series, mounted on a bracing support frame and provided with a fail-safe inflation system.

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