A model of major fire spread in a tunnel is described. It employs the concepts of non-linear dynamical systems theory and identifies the onset of instability with major fire spread in a tunnel. In particular, the model associates the existence of a fold bifurcation with dramatic fire spread from an initial fire to a 'target object'. The purpose is to identify the thermo-physical and geometrical conditions which lead to instability and sudden fire spread. Flame impingement on the target object is assumed not to exist; fire spread is assumed to be by spontaneous ignition only. The case considered assumes the existence of a longitudinal forced ventilation and predicts the critical heat release rate needed for a fire to spread from an initial fire to an item with a given assumed shape. The target object may be taken to approximate a vehicle. The illustrative case approximating fire spread from an initial fire to a heavy goods vehicle (HGV) within the Channel Tunnel is presented; it is not restricted to this case, however. The model is identified with the name FIRE-SPRINT A3, which is an acronym of Fire Spread in Tunnels, Model A, Version 3. It is a development of an earlier model, FIRE-SPRINT A2. The current model takes greater account of thermal radiation than was done in the earlier work and also assumes a more extensive flame volume for a downstream flame section. © Springer Science + Business Media, LLC. 2006.