The ICOS Central Analytical Laboratories (CALs) play a central role in assuring the accuracy of atmospheric observations within ICOS. This involves the central provision of reference gases to the ICOS atmospheric network and calibrating these standards based on the World Meteorological Organization (WMO) calibration scales. A quality control strategy for the ICOS atmospheric measurements has been described within the Atmospheric Station Specification document [ATC 2020]. In this report the quality control measures are described that are made by the ICOS-CAL Flask and Calibration Laboratory (FCL) to characterize the performance of their calibration of ICOS reference gases. It updates and replaces the QC 2020 report following the same assessment scheme with only minor changes and some few corrections. The results of these activities of the recent years are presented in detail for each of the ICOS core components for in-situ observations (CO2, CH4, CO) and N2O. The results are then assessed and used to substantiate estimates of the measurement uncertainties of the different tracers and to quantify different uncertainty contributions. This involves an evaluation of the uncertainty of the reference values of calibration standard gases ("scale link uncertainty") and the measurement uncertainty related to the respective analyzer’s precision or response stability over time.