A reference key based on the size of the population and of the GDP of the Member States should be applied in accordance with the mandatory fair share principle for the operation of the solidarity mechanism enabling the determination of the overall contribution of each Member State. A Member State could, on a voluntary basis, provide an overall contribution beyond its mandatory fair share to assist Member States under migratory pressure. In the operationalisation of the Annual Solidarity Pool, contributing Member States should implement their pledges in proportion to their overall pledge, so that each time solidarity is drawn from the pool, those Member States contribute according to their fair share. In order to safeguard the functioning of this Regulation, the contributing Member States should not be obliged to implement their solidarity pledges towards the benefitting Member State where the Commission has identified systemic shortcomings in that benefitting Member State with regard to the rules set out in Part III of this Regulation that could result in serious negative consequences for the functioning of this Regulation.