Abstract The constancy of the speed of light c is the foundation of most physical theories and is found to be indeed constant from local measurements. Whether or not the same is true at cosmological timescale has been of interest for some time. The measurement of time delay difference in the peaking of the luminosity between two gravitationally lensed images of a type 1a-supernova has recently been proposed to determine if c was different from its current value at the time the images were produced by a lensing galaxy. Since the geometrical time delay and the Shapiro time delay both scale as G/c 3 we show that this method of constraining c is valid only when the potential variation of the gravitational constant G is ignored: c variation is imperceptible when G ∼ c 3.