When a flammable gas or vapour is released into an area which is congested (e.g. a typical process plant) and forms a flammable cloud which is subsequently ignited, a vapour cloud explosion can occur. Large-scale experimental studies have previously shown that the presence of obstacles within a flammable cloud can lead to flame acceleration, resulting in flame speeds that are capable of producing damaging overpressures. This work also showed that ignition of a flammable gas-air mixture within a confined region, such as a building, that results in an explosion that vents into an external flammable cloud containing congestion, can generate high flame speeds and severe overpressures, with deflagration to detonation transition occurring in some cases. As few published large scale studies have considered the effects of a confined explosion venting into an obstructed flammable vapour cloud, this paper discusses a comprehensive large-scale experimental study carried out by the former British Gas Research and Development (now GL Noble Denton) on the effect that a number of parameters have on the overpressures generated by a confined explosion venting into an external flammable cloud. The results show that the presence of even relatively small volumes of pipework congestion significantly increases the measured external overpressures. In addition, there is clear evidence that for ethylene-air and propane-air mixtures, deflagration to detonation transition occurred as the flame exited from the external congested region. This has particular relevance to two recent vapour cloud explosion incidents at Buncefield, UK and Jaipur, India, where the evidence is consistent with detonation of most of the vapour cloud.