  Magnetar power is believed to be at the origin of numerous super-luminous
supernovae (SNe) of Type Ic, arising from compact, hydrogen-deficient,
Wolf-Rayet type stars. Here, we investigate the properties that magnetar power
would have on standard-energy SNe associated with 15-20Msun blue or red
supergiant (BSG/RSG) stars. We use a combination of Eulerian grey
radiation-hydrodynamics and non-LTE steady-state radiative transfer to study
their dynamical, photometric, and spectroscopic properties. Adopting magnetar
fields of 1, 3.5, 7 x 10^14G and rotational energies of 0.4, 1, and 3 x
10^51erg, we produce bolometric light curves with a broad maximum covering
50-150d and a magnitude of 10^43-10^44erg/s. The spectra at maximum light are
analogous to those of standard SNe II-P but bluer. Although the magnetar energy
is channelled roughly in equal proportion between SN kinetic energy and SN
luminosity, the latter may be boosted by a factor 10-100 compared to a standard
SN II. This influence breaks the observed relation between brightness and
expansion rate of standard Type II SNe. Magnetar energy injection also delays
recombination and may even cause re-ionization, with a reversal in photospheric
temperature and velocity. Depositing the magnetar energy in a narrow mass shell
at the ejecta base leads to the formation of a dense shell at a few 1000km/s,
which causes a light-curve bump at the end of the photospheric phase.
Depositing this energy over a broad range of mass in the inner ejecta, to mimic
the effect of multi-dimensional fluid instabilities, prevents the formation of
a dense shell and produces an earlier-rising and smoother light curve. The
magnetar influence on the SN radiation is generally not visible prior to
20-30d, during which one may discern a BSG from a RSG progenitor. We propose a
magnetar model for the super-luminous Type II SN OGLE-SN14-073.
