Work on modernizing the Cruiser Tank Mk.III began in 1938, the same year as it was accepted into service. The Cruiser Tank A13E3 became a test lab for ideas for the new vehicle. No serious changes would be made to the hull, suspension, or engine, all of these were satisfactory to the British. Changes were confined to the armour, and applique armour at that. The only part that became thicker was the driver's hatch, increased to 28 mm.
The turret had some changes done to it. The commander's cupola became round instead of hexagonal (similar to the cupola of the Light Tank Mk.VIB). The observation device to the left of the gun mantlet was moved to the applique armour, and the four sections of front applique armour were installed parallel to the hull. The configuration of the side applique armour was similar to that used on the Cruiser Tank Mk.III*. The gap that was characteristic for field modifications disappeared.
- T.7030-T.7094 (65 tanks)
- T.9096-T.9190 (95 tanks)
- T.15215-T.15294 (80 tanks)
- T.18096-T.18160 (65 tanks)
- 2nd Armoured Brigade: 2nd Dragoon Guards Regiment, 9th Her Majesty's Own Royal Lancers Regiment, and 10th Royal Hussars Regiment.
- 3rd Armoured Brigade: 2nd, 3rd, and 5th Royal Armoured Regiments.