According to Tank Encylopedia’s ‘The Online Tank Museum’ -
In the late 1940s, the British War Office (WO) was concerned that – after the debut of the IS-3 in 1945 – the Soviet Union would continue to develop heavily armored tanks. As such, the War Office filed a requirement for the development of a gun capable of defeating a 60-degree sloped plate, 6 inches (152 mm) thick, at up to 2,000 yards (1,830 meters), and a suitable vehicle to carry it.
This requirement led to the development of the ‘Ordnance, Quick-Firing, 183 mm, Tank, L4 Gun’, the largest purpose-built anti-tank gun to have ever been created. It was intended that this gun would be mounted on a new ‘Heavy Gun Tank’ based on the FV200 series chassis. This was designated the ‘Tank, Heavy No. 2, 183 mm Gun, FV215’.