In this paper the results from the first series of JET experiments dedicated to the investigation of the properties of the H-mode power threshold are presented. The experiments were carried out in an ITER-like magnetic equilibrium and made use of the Mark I Pumped Divertor. Scaling of the threshold power is investigated in terms of both global variables and local edge parameters. First evidence of the existence of a minimum edge temperature (Tcrit) necessary for the L-H transition is reported. Deviation from the linear density scaling of the threshold power, observed at ne > 5x1019 m-3 (70% of the Greenwald limit at 3MA) is shown to be due to detachment and MARFE-like structures at the divertor during the L-mode phase, the effect of which is to keep the plasma edge cool and Te < Tcrit.