We have obtained J photometry for a sample of 12 objects in the interval 16.0 < I <= 20.5 which are member candidates in the young σ Orionis cluster. Our data confirms that 11 of them do fit the photometric sequence expected for the cluster implying their very likely membership in σ Orionis. Their masses span from 0.09 Modot down to the very low-mass brown dwarf regime at the very young age (1--5 Myr) of the cluster. We have also obtained low-resolution spectroscopy for the faintest object in our sample, SOri 47 (I = 20.5). We classify it as an early L-type young brown dwarf with a very likely mass around the deuterium burning limit. SOri 47 provides evidence that the initial mass function extends down to masses of the order of 0.015 Modot and serves as a guide for the study of the new L-class discoveries in the field.