We present a modular rack-mounted laser system for the cooling and manipulation of neutral rubidium atoms which has been developed for a portable gravimeter based on atom interferometry that will be capable of performing high-precision gravity measurements directly at sites of geophysical interest. This laser system is constructed in a compact and mobile design so that it can be transported to different locations, yet it still offers improvements over many conventional laboratory-based laser systems. Our system is contained in a standard 19" rack and emits light at five different frequencies simultaneously on up to 12 fibre ports at a total output power of 800 mW. These frequencies can be changed and switched between ports in less than a microsecond. The setup includes two phase-locked diode lasers with a phase noise spectral density of less than 1 μrad/Hz1/2 in the frequency range in which our gravimeter is most sensitive to noise. We characterise this laser system and evaluate the performance limits it imposes on an interferometer