One of the main concerns about the use of fibre reinforced polymer (FRP) materials in civil engineering structural applications is the potential reduction of their stiffness- and strength-related properties when exposed to elevated temperature. The first part of this paper presents an assessment of existing design guidelines for FRP structures, regarding the recommendations they provide about the reduction of FRP mechanical properties at elevated temperature. The second part of this paper presents a survey of test data available in the literature concerning mechanical tests at elevated temperature in FRP materials produced with different fibres, resins, shapes and manufacturing methods. The results of the survey show that the provisions available in existing design guidelines are not always accurate; moreover, in some cases, they are even non-conservative. Based on this assessment, the third part of the paper presents a method to define a temperature conversion factor for design purposes, consistent with the partial factor method of the Eurocodes, which was calibrated with the results included in the database. The method proposed takes into account not only the maximum service temperature experienced by the FRP material, but also its glass transition temperature; the method considers also the type of mechanical property, namely if it is either fibre- or matrix-dominated. The conversion factors obtained applying the method proposed presented a good agreement with the test data available in the literature.