We report the results of a high-pressure high-temperature (HPHT) experimental investigation into the deformation of diamonds using the D-DIA apparatus. Electron backscatter diffraction (EBSD) data confirm that well-defined 300-700 nm wide {111} slip lamellae are in fact deformation micro-twins with a 60 degrees rotation around a < 111 > axis. Such twins formed at high confining pressures even without any apparatus-induced differential stress; mechanical anisotropy within the cell assembly was sufficient for their formation with very little subsequent lattice bending (<1 degrees per 100 mu m). When apparatus-induced differential stresses were applied to diamonds under HPHT conditions, deformation twin lamellae were generated, and continuous and discontinuous crystal lattice bending occurred (4-18 degrees per 100 mu m), including bending of the {111} twin lamellae. The (111) < 011 > slip system dominates as expected for the face-centred cubic (FCC) structure of diamond. Slip occurs on multiple {111} planes resulting in rotation around < 112 > axes. Deformation microstructure characteristics depend on the orientation of the principal stress axes and finite strain but are independent of confining pressure and nitrogen content. All of the uniaxially deformed samples took on a brown colour, irrespective of their initial nitrogen characteristics. This is in contrast to the two quasi-hydrostatic experiments, which retained their original colour (colourless for nitrogen free diamond, yellow for single substitutional nitrogen, Type lb diamond) despite the formation of (111) twin lamellae. Comparison of our experimental data with those from two natural brown diamonds from Finsch mine (South Africa) shows the same activation of the dominant slip system. However, no deformation twin lamellae are present in the natural samples. This difference may be due to the lower strain rates experienced by the natural samples investigated. Our study shows the applicability and potential of this type of analysis to the investigation of plastic deformation of diamonds under mantle conditions.