We present the final results from the XMM-Newton validation follow-up of new Planck galaxy cluster candidates . We observed 15 new candidates , detected with signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck survey . The candidates were selected using ancillary data flags derived from the ROSAT All Sky Survey ( RASS ) and Digitized Sky Survey all-sky maps , with the aim of pushing into the low SZ flux , high- z regime and testing RASS flags as indicators of candidate reliability . Fourteen new clusters were detected by XMM-Newton , ten single clusters and two double systems . Redshifts from X-ray spectroscopy lie in the range 0.2 to 0.9 , with six clusters at z > 0.5 . Estimated masses ( M _ { 500 } ) range from 2.5 \times 10 ^ { 14 } to 8 \times 10 ^ { 14 } \thinspace { M _ { \odot } } . We discuss our results in the context of the full XMM-Newton validation programme , in which 51 new clusters have been detected . This includes four double and two triple systems , some of which are chance projections on the sky of clusters at different redshifts . We find that association with a source from the RASS-Bright Source Catalogue is a robust indicator of the reliability of a candidate , whereas association with a source from the RASS-Faint Source Catalogue does not guarantee that the SZ candidate is a bona fide cluster . Nevertheless , most Planck clusters appear in RASS maps , with a significance greater than 2 \sigma being a good indication that the candidate is a real cluster . Candidate validation from association with SDSS galaxy overdensity at z > 0.5 is also discussed . The full sample gives a Planck sensitivity threshold of Y _ { 500 } \sim 4 \times 10 ^ { -4 } arcmin ^ { 2 } , with indication for Malmquist bias in the Y _ { X } – Y _ { 500 } relation below this threshold . The corresponding mass threshold depends on redshift . Systems with M _ { 500 } > 5 \times 10 ^ { 14 } \thinspace { M _ { \odot } } at z > 0.5 are easily detectable with Planck . The newly-detected clusters follow the Y _ { X } – Y _ { 500 } relation derived from X-ray selected samples . Compared to X-ray selected clusters , the new SZ clusters have a lower X-ray luminosity on average for their mass . There is no indication of departure from standard self-similar evolution in the X-ray versus SZ scaling properties . In particular , there is no significant evolution of the Y _ { X } / Y _ { 500 } ratio .