Systematic uncertainties associated to type Ia supernova (SN Ia) Hubble diagrams from
photometrically selected samples using photometric SN host galaxy redshifts are investigated. The
host redshift uncertainties and the contamination by core-collapse SNe are both addressed. As a
test case, we use the 3-year photometric SN Ia sample of the SuperNova Legacy Survey (SNLS),
consisting of 437 objects between 0.1 and 1.05 in redshift with 4.7% contamination. We combine
this sample with non-SNLS objects of the spectroscopic sample from the joint analysis (JLA) of the
SDSS-II and SNLS collaborations, consisting of 501 objects mostly below 0.4 in redshift. We study
two options for the origin of the redshifts of the photometric sample, either provided entirely
from the host photometric redshift catalogue used in the selection or a mixed origin where around
75% of the sample can be assigned spectroscopic redshifts from dedicated measurements or
external catalogues. Using light curve simulations subject to the same photometric selection as
data, we study the impact of photometric redshift uncertainties and contamination on flat
ΛCDM fits to Hubble diagrams from such combined samples. Our primary finding is that photometric
redshifts and contamination lead to biased cosmological parameters. The magnitude of the bias is
found to be similar for both redshift options. This bias can be largely accounted for if
photometric redshift uncertainties and contamination are taken into account when computing the SN
magnitude bias correction due to selection effects. To reduce the residual cosmological bias, we
explore two methods to propagate redshift uncertainties into the cosmological likelihood
computation, either by refitting photometric redshifts along with cosmology or by sampling the
redshift resolution function. Redshift refitting fails at correcting the cosmological bias
whatever the redshift origin, while sampling slightly reduces it in both cases. Finally, for
actual data, we find compatible results with those from the JLA diagram for mixed photometric and
spectroscopic redshifts, while the full photometric option is biased upwards, but consistent with
JLA when all statistical and systematic uncertainties are included.