Synoptic messages (SYNOP) exchanged internationally for operational weather forecasting are regularly used to extend validated (quality controlled) daily climate time series to the present day, despite differences in measuring intervals and lack of validation. Here we focus on the effect of this on derived climate indices of extremes in Europe. Validated time series are taken from the European Climate Assessment & Dataset (ECA&D). Validated data and SYNOP over the period 01 April 1982 to 31 December 2004 are compared. The distribution of the difference series of validated data and SYNOP is skewed. Generally, minimum temperatures are lower or equal in the validated series, while maximum temperatures are higher or equal. This is at least partly due to the 24‐hour (validated data) versus 12‐hour (SYNOP) measuring intervals. The precipitation results are dependent on the difference between the measuring intervals of both time series. Time series of indices of extremes exhibit a non‐climatic inhomogeneity for several indices when SYNOP are used to extend the validated series, leading to spurious trends. The sizes of the trends in pure validated and pure SYNOP series are generally in good agreement, but the absolute values of the indices show an offset. Accepting a trend error of 10%, the averaged winter minimum and maximum temperature and the number of tropical nights (minimum temperature >20°C) in summer allow only a very small fraction of SYNOP in the extended series (about 5–10%), while for the other indices studied here a larger fraction can be used (up to 50%).