Abstract : We present the XMM-Newton follow-up for validation of Planck cluster candidates. Twenty-five candidates have been observed to date using snapshot (~10 ksec) exposures: ten as part of a pilot programme to sample a low range of signal-to-noise ratios (45 candidates. The sensitivity and spatial resolution of XMM-Newton allows unambiguous discrimination between clusters and false candidates. A total of 21 candidates are confirmed as extended X-ray sources. Seventeen are single clusters, the majority of which are found to have highly irregular and disturbed morphologies. The remaining four sources are multiple systems, including the unexpected discovery of a supercluster at z=0.45. For most of the sources we are able to derive a redshift estimate from the X-ray Fe K line (albeit of variable quality). The new clusters span the redshift range 0.09 <~ z <~ 0.54 with a median redshift of z ~ 0.37. A first estimate is made of their X-ray properties including the characteristic size, which is used to improve the SZ Compton parameter estimate. The validation programme has helped to optimise the Planck candidate selection process. It has also provided a preview of the X-ray properties of these newly-discovered clusters, allowing comparison to their SZ properties, and to the X-ray and SZ properties of known clusters observed in the Planck survey. Our results suggest that Planck may have started to reveal a non-negligible population of massive dynamically perturbed objects that is under-represented in X-ray surveys. However, despite their particular properties, these new clusters appear to follow the Ysz-Yx relation established for X-ray selected objects.