A pneumatic valve for attachment to a source of high pressure gas comprises a valve body which comprises a gas inlet and a gas outlet. A rupturable membrane extends across the gas inlet. A membrane support element is slidably supported in the valve body for movement between an extended position and a retracted position, the membrane support element having a lower end engageable, in the extended position, with the rupturable membrane to support the rupturable membrane against rupture. The valve body further comprises a bore through which the membrane support element extends and one or more gas supply passages bypassing the bore for providing a gas flow path from the gas inlet to the gas outlet. The membrane support element is configured to be movable from the extended position to the retracted position to permit the rupturable membrane to rupture under the pressure of the high pressure gas.