Context : Thanks to the outstanding capabilites of the HST , our current knowledge about the M31 globular clusters ( GCs ) is similar to our knowledge of the Milky Way GCs in the 1960s-1970s , which set the basis for studying the halo and galaxy formation using these objects as tracers , and established their importance in defining the cosmic distance scale .

Aims : We intend to derive a new calibration of the M _ { V } ( HB ) - [ Fe/H ] relation by exploiting the large photometric database of old GCs in M31 in the HST archive .

Methods : We collected the BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects .
We obtained a set of homogeneous colour-magnitude diagrams ( CMDs ) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs .
Reddening , metallicity , Horizontal Branch ( HB ) luminosity and distance were determined self-consistently for each cluster .

Results : There are three main results of this study : i ) the relation M _ { V } ( HB ) =0.25 ( \pm 0.02 ) [ Fe/H ] +0.89 ( \pm 0.03 ) , which is obtained from the above parameters and is calibrated on the distances of the template Galactic GCs ; ii ) the distance modulus to M31 of ( m-M ) _ { 0 } =24.42 \pm 0.06 mag , obtained by normalising this relation at the reference value of [ Fe/H ] =–1.5 to a similar relation using V _ { 0 } ( HB ) .
This is the first determination of the distance to M31 based on the characteristics of its GC system which is calibrated on Galactic GCs ; iii ) the distance to the Large Magellanic Cloud ( LMC ) , which is estimated to be 18.54 \pm 0.07 mag as a consequence of the previous results .
These values agree excellently with the most recent estimate based on HST parallaxes of Galactic Cepheid and RR Lyrae stars , as well as with recent methods .

Conclusions :