[en] We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV⁻) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV⁻ centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV⁻–based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometer (LTM). We predict that an NV⁻–based LTM with a volume of 1 mm³ can achieve shot-noise limited dc sensitivity of 1.86 fT $/\sqrt{\mathrm{H}\mathrm{z}}$ and ac sensitivity of 3.97 fT $/\sqrt{\mathrm{H}\mathrm{z}}$. (paper)