We used a \sim 300 ks long XMM-Newton  observation of the Seyfert 1 galaxy MCG–6-30-15 to study the correlation between the 0.2–10 keV X-ray and the 3000–4000 à  U bands .
We found a significant correlation peak at a time lag of \tau _ { max } \sim 160 ks where the UV flux variations preceded the variations in the X-ray band .
We interpret this result as evidence in favour of Comptonisation models where the observed X-rays are produced through Compton up-scattering of thermal UV seed photons from an accretion disc , as this process naturally predicts the UV variations to precede similar flux variations in the X-rays .
The length of the time lag favours models where the observed UV and the seed-photon-emitting regions are connected by perturbations of the accretion flow traveling inwards through the disc , affecting first the main- U -band-emitting radii and then the innermost region where the bulk of the seed photons is expected to be produced .
Finally , the absence of significant features in the correlation function with X-ray flux variations preceding those in the UV indicates that the observed U -band photons are not mainly produced through reprocessing of hard X-rays in this source .