The invention relates to injectable long-acting insulin formulations for the treatment of type I and II diabetes in humans and animals. The main objective of the invention is to provide a long-acting insulin formulation in the form of a colloidal suspension: which allows easy filling of a syringe through a small diameter needle (for example with the gauge 29 G, 30 G or 31 G) and/or which can be easily injected through a small diameter needle (for example with the gauge 29 G, 30 G or 31 G),without damaging the therapeutic efficacy of the insulin. To achieve this objective, the subject of the invention is an aqueous and stable colloidal formulation of nanoparticles of at least one poly(Leu-block-Glu), loaded with insulin, in which the pH is such that: 6.0.ltoreq.pH.ltoreq.7.0 which comprises at least one magnesium salt in a quantity such that: the osmolarity Osm (in mOsmol) is such that: 270.ltoreq.Osm.ltoreq.600, the viscosity v (in mPas), measured according to a procedure Mv, is such that: v.ltoreq.15; the poly(Leu-block-Glu) concentration (in mg/ml) is between 30 and 70, preferably between 38 and 65.