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We describe the construction of an X-ray flux-limited sample of galaxyclusters, the REFLEX survey catalogue, to be used for cosmological studies. Thesurvey is based on the ROSAT All-Sky Survey and on extensive follow-upobservations conducted in the frame of an ESO key program. In the paper wedocument the process by which the primary cluster candidate catalogue isconstructed prior to the optical follow-up observations. We describe thereanalysis of the RASS source catalogue which enables us to impose a properflux limit cut to the X-ray source list without introducing a severe biasagainst extended sources. We discuss the correlation of the X-ray and optical(COSMOS) data to find galaxy density enhancements at the RASS X-ray sourcepositions and the further evaluation of the nature of these cluster candidates.Based also on the results of the follow-up observations we provide astatistical analysis of the completeness and contamination of the final clustersample and show results on the cluster number counts. The final sample ofidentified X-ray clusters reaches a flux limit of 3e-12 erg s-1 cm-2 in the 0.1- 2.4 keV band and comprises 452 clusters. The results imply a completeness ofthe REFLEX cluster sample well in excess of 90% and a contamination bynon-cluster X-ray sources of less than 9%, an accuracy sufficient for the useof this cluster sample for cosmological tests.Comment: 24 pages, 31 figures, submitted to Astronomy and Astrophysic

The ROSAT-ESO flux limited X-ray (REFLEX) galaxy cluster survey. I. The construction of the cluster sample