We report the results of XMM-Newton observations of the X-ray bright , radio-loud blazar GB B1428+4217 at a redshift of z = 4.72 .
We confirm the presence of soft X-ray spectral flattening at energies \lesssim 0.7 { \thinspace keV } as reported in previous ROSAT and BeppoSAX observations .
At hard X-ray energies the spectrum is consistent with a power-law although we find the spectral slope varied between both XMM-Newton observations and is also significantly different from that reported previously .
Whilst we can not rule-out intrinsic cold absorption to explain the spectral depression , we favour a dust-free warm absorber .
Cold absorption requires a column density \sim 1.4 - 1.6 \times 10 ^ { 22 } \hbox { $ \thinspace cm ^ { -2 } $ } whilst a warm absorber could have up to \sim 10 ^ { 23 } \hbox { $ \thinspace cm ^ { -2 } $ } and an ionization parameter \sim 10 ^ { 2 } .
The spectrum of GB B1428+4217 shows remarkable parallels with that of the z = 4.4 blazar PMN J0525–3343 , in which the available evidence is also most consistent with a warm absorber model .