Experiments in JET with the Be/W ITER-like wall show that in pure hydrogen low-confinement mode (L-mode) plasmas the density limit is approximately 20% higher than their corresponding deuterium plasmas. The maximum achievable density in L-mode plasmas is limited by the magnetohydrodynamic stability of the m/n = 2/1 tearing mode. Studies with the edge fluid-neutral Monte-Carlo code package EDGE2D-EIRENE show that the density of hydrogen atoms inside the separatrix is two times lower than for deuterium in plasma conditions preceding the density limit. The difference between the isotopes is caused by the non-linear process at density limit onset which leads to more efficient dissociation and ionization of hydrogen molecules and atoms in hydrogen than in deuterium plasmas at the high-field X-point region at electron temperatures lower than 2 eV. The m/n = 2/1 island size is estimated to be 30% smaller islands for hydrogen than for deuterium cases for equal fuelling conditions and radial transport assumptions.