The Chieftain 900 was a British main battle tank developed as a modernisation of the original Chieftain in the 1980s. The project was initiated by Royal Ordnance Factories (ROF) in an attempt to address the tank’s main shortcomings — poor mobility, an underpowered engine, and outdated armour protection. The modernisation plan included the installation of Chobham armour, a new 900 hp Rolls-Royce Condor diesel engine, upgraded suspension and transmission, and improved fire-control electronics. Two Iranian export Chieftain hulls, left undelivered after the fall of the Shah in 1978, were used to build the prototypes. The first vehicles were completed in 1982 and displayed at the British Army Equipment Exhibition. However, the exhibited tanks only featured mock-up Chobham armour, despite ROF’s promotional claims. With no domestic or foreign orders, the project was discontinued in 1986.

The Chieftain 900 was introduced in Update “Spearhead”. It successfully eliminates the greatest weakness of the earlier Chieftain Mk.3 and Mk.5 — their poor mobility. The new tank features a 900 hp engine, resulting in improved acceleration and top speed, a laser rangefinder integrated into the fire-control system, and access to the L23 APFSDS round. However, the vehicle still suffers from a vulnerable lower glacis and insufficient protection of both the hull and turret.