The gas industry has an excellent safety record in operating high pressure transmission pipelines. Nevertheless, it is important that pipeline operators have an understanding of the possible consequences of an accidental gas release, which may ignite, in order to help manage the risks involved and to develop appropriate standards and design codes for pipeline operations. The most serious type of hypothetical incident that could affect a high pressure gas transmission pipeline, involves a full pipe rupture. This paper describes the extension and validation of a physically-based phenomenological model of jet fires for predicting the size and position of the fires resulting from underground gas pipeline ruptures and to predict thermal radiation levels around such fires. The application of the complete model to underground pipeline ruptures has been made possible by the development of a crater source sub-model which provides suitable input parameters to the fire structure calculation. Predictions of the complete model have been compared with the results from two full-scale pipeline rupture tests, which has demonstrated the performance of the model for a range of realistic scenarios. The work was undertaken by an international collaboration of gas companies, as part of a programme of research to obtain information on the consequences of accidental gas releases from transmission pipelines.