This work is aimed at the investigation of the spin relaxation in controllably charged quantum dots (QDs). Measuring the dependence of the spin lifetime on the gate voltage in a heterostructure containing InP QDs, we detected the onset of spin conservation when the QD became negatively charged. The spin lifetime in the charged QDs exceeded 1 ns. Various properties of the long-lived spin polarisation of the QDs in longitudinal magnetic field as well as under various excitation conditions are discussed.