We discuss the construction of an X-ray flux-limited sample of galaxy clusters , the REFLEX survey catalogue , to be used for cosmological studies .
This cluster identification and redshift survey was conducted in the frame of an ESO key programme and is based on candidates selected from the southern part of the ROSAT All-Sky Survey ( RASS ) .
For the first cluster candidate selection from a flux-limited RASS source list we make use of optical data from the COSMOS digital catalogue produced from the scans of the UK-Schmidt plates .
To ensure homogeneity of the sample construction process this selection is based only on this one well defined optical data base .
The nature of the candidates selected in this process is subsequently checked by a more detailed evaluation of the X-ray and optical source properties and available literature data .
The final identification and the redshift is then based on optical spectroscopic follow-up observations .

In this paper we document the process by which the primary cluster candidate catalogue is constructed prior to the optical follow-up observations .
We describe the reanalysis of the RASS source catalogue which enables us to impose a proper flux limit cut to the X-ray source list without introducing a severe bias against extended sources .
We discuss the correlation of the X-ray and optical ( COSMOS ) data to find galaxy density enhancements at the RASS X-ray source positions and the further evaluation of the nature of these cluster candidates .
Based also on the results of the follow-up observations we provide a statistical analysis of the completeness and contamination of the final cluster sample and show results on the cluster number counts .
The final sample of identified X-ray clusters reaches a flux limit of 3 \cdot 10 ^ { -12 } erg s ^ { -1 } cm ^ { -2 } in the 0.1 - 2.4 keV band and comprises 452 clusters .
The results imply a completeness of the REFLEX cluster sample well in excess of 90 % and a contamination by non-cluster X-ray sources of less than 9 % , an accuracy sufficient for the use of this cluster sample for cosmological tests .