One of the main concerns about the use of fibre-reinforced polymer (FRP) materials in civil engineering struc-tures is the potential reduction of their stiffness-and strength-related properties when exposed to elevated temperature. This concern, mostly due to the glass transition process underwent by polymeric matrices, needs to be duly reflected in design standards. In the scope of the development of the European Technical Specification CEN/TS 19101: 2022, "Design of Fibre-Polymer Composite Structures", this paper presents the definition of a temperature conversion factor for in-service conditions to take into account the reduction of FRP mechanical properties with temperature. The first part of the paper briefly describes the basis of design of CEN/TS 19101: 2022 and the framework to consider the effects of environmental conditions on material properties. The second part presents an assessment of recommendations about the reduction of mechanical properties with temperature in existing design guidelines for FRP structures. The third part describes a survey of test data available in the literature concerning mechanical tests at elevated temperature of FRP materials produced with different fibres, resins, shapes and manufacturing methods. The survey shows that the provisions available in existing design guidelines are not always precise and, in some cases, they are non-conservative. Based on this assessment, the fourth and final part of the paper presents the method used to define the values of the temperature conversion factor included in CEN/TS 19101: 2022, which is consistent with the partial factor method of the Eurocodes. The method takes into account (i) the maximum service temperature experienced by the FRP material, (ii) its glass transition temperature, and (iii) the type of mechanical property, namely if it is either fibre-or matrix -dominated. The proposed temperature conversion factor values proved to be adequate considering the test data available in the literature.