Polymeric materials are increasingly used in several applications. However, their relatively high flammability presents a danger to people and property. Their use therefore requires that they are made more resistant to both the initiation and propagation of fires. In the present work we evaluate, by cone calorimeter tests, the efficiency of urea complex of magnesium and vermiculite (urea- vermiculite) as a stand-alone flame retardant in flexible polyvinyl chloride (PVC) and polyurethane (PU) resin. Flexible PVC cone calorimeter tests proceeded without a visible flame following an initial short-lived ignition which degenerated into a bulk pyrolysis, in combination with a surface glowing-combustion event. Urea-vermiculite formed an exfoliated protective barrier layer which allowed thermal stabilisation of the condensed phase. The simultaneous release of halogen species by the PVC and the action of the exfoliated barrier layer prevented the formation of a flammable air–fuel mixture. The addition of urea-vermiculite lowered the peak heat release rate (pHRR) and the total heat released (tHR) significantly. In PU composites the urea-vermiculite was unable to form a cohesive protective barrier layer during the cone calorimeter test. The poor compatibility between the molten PU and the exfoliated flakes also led to the consumption of the underlying PU. Nevertheless, the addition of urea-vermiculite lowered significantly the pHRR of PU composites. In general, cone calorimetry results revealed that urea-vermiculite allowed thermal stabilization of the condensed phase at high temperatures but had little influence in the vapour phase behaviour. The amount of released urea’s degradation products (non-flammable vapours) was not enough to dilute the flammable vapours’ mixture. Thereby when used with PVC, which releases halogen flame poison, it showed great fire performance.