  Type Ia supernovae (SNIe) are generally accepted to act as standardisable
candles, and their use in cosmology led to the first confirmation of the as yet
unexplained accelerated cosmic expansion. Many of the theoretical models to
explain the cosmic acceleration assume modifications to Einsteinian General
Relativity which accelerate the expansion, but the question of whether such
modifications also affect the ability of SNIe to be standardisable candles has
rarely been addressed. This paper is an attempt to answer this question. For
this we adopt a semi-analytical model to calculate SNIe light curves in
non-standard gravity. We use this model to show that the average rescaled
intrinsic peak luminosity -- a quantity that is assumed to be constant with
redshift in standard analyses of Type Ia supernova (SNIa) cosmology data --
depends on the strength of gravity in the supernova's local environment because
the latter determines the Chandrasekhar mass -- the mass of the SNIa's white
dwarf progenitor right before the explosion. This means that SNIe are no longer
standardisable candles in scenarios where the strength of gravity evolves over
time, and therefore the cosmology implied by the existing SNIa data will be
different when analysed in the context of such models. As an example, we show
that the observational SNIa cosmology data can be fitted with both a model
where $(\Omega_{\rm M}, \Omega_{\Lambda})=(0.62, 0.38)$ and Newton's constant
$G$ varies as $G(z)=G_0(1+z)^{-1/4}$ and the standard model where $(\Omega_{\rm
M}, \Omega_{\Lambda})=(0.3, 0.7)$ and $G$ is constant, when the Universe is
assumed to be flat.
