The paper gives guidance on the use of the EC2-1-2 model of degradation of the mechanical properties of prestressing steel at high temperature useful for estimating the loss of prestressing force in members exposed to fire. Adequate estimation of this force is crucial when calculating the fire resistance of prestressed members. If only the reduction in steel strength determined on the basis of the EC2-1-2 model is considered for the analysis, it may cause a significant underestimation of the prestress loss under fire conditions. In order to adequately estimate this force, it is necessary to sum up the strain of the prestressing tendons resulting from the degradation of their mechanical properties under the influence of high temperature and the free thermal elongation of the steel. The paper points out the existence of analogies between the elongation of prestressing steel heated under fire conditions and concrete creep under ordinary conditions. The guidelines for estimating thermal stress relaxation in tendons and determining the reduction in the secant modulus of prestressing steel under fire conditions are also provided.