  We present the XMM-Newton follow-up for confirmation 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 (4<S/N<6), and a further 15 in a programme to observe a
sample of S/N>5 candidates. The sensitivity and spatial resolution of
XMM-Newton allows unambiguous discrimination between clusters and false
candidates. The 4 false candidates have S/N <= 4.1. 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
(about ~70%). The remaining four sources are multiple systems, including the
unexpected discovery of a supercluster at z=0.45. For 20 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 determination is made of their X-ray
properties including the characteristic size, which is used to improve the
estimate of the SZ Compton parameter, Y_SZ. The follow-up 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 with 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 Y_SZ-Y_X relation established for X-ray selected objects, where Y_X
is the product of the gas mass and temperature.
