The flame folding, induced by the classical Darrieus-Landau instability, is identified as a possible mechanism for the spontaneous transition from deflagration to detonation. A numerical simulation of premixed gas combustion spreading from the closed end of a semi-infinite, smooth-walled channel is performed. It is found that in sufficiently wide channels the Darrieus-Landau instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. The mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation. The transition is found to be sensitive to the flame’s incipient speed and the reaction rate molecularity, favoring fast flames and high order reactions. To gain a better insight into the mechanism of the transition, a one-dimensional model is explored whose basic characteristics are borrowed from the two-dimensional system. Zel’dovich’s theory of soft initiation, requiring the fresh mixture to be appropriately preconditioned, is substantiated.