The Daimler DK400, originally called the Daimler Regina, was a large luxury car made by Daimler Motor Company between 1955 and 1959. It was generally equipped with steel limousine bodywork by Carbodies DK400B with a drop division, three occasional seats and more luxurious trim in the rear compartment than in the DK400A owner-driver car. Some chassis were bodied as hearses and at least one was equipped with a coupé body for Lady Docker. An unusual feature of the limousine was that the occasional middle seats were three abreast.
The DK400 was the last new Daimler model to use the company's preselector gearbox with fluid flywheel transmission introduced in 1930. It was replaced in the Majestic by a more modern Borg-Warner fully automatic transmission. A major change in gearing, however, saw the direct-drive top gear replaced by a direct-drive third and an overdrive top. This, intended to improve the car's ability to cruise at high speed with its large engine, could lead to failure because the transmission could not always cope with the torque available from an engine capable of producing 167 horsepower (125 kW) at 3,000 rpm.
It also saw the furthest development of the old long-stroke (95.2 millimetres (3.75 in) bore and 107 millimetres (4.2 in) stroke) pushrod straight six engines that had been traditional on Daimler's smaller cars since 1910. They gave way to the modern and lightweight V8s in the last Daimler engines designed and put into production before Jaguar bought the company in 1960. As a consequence, when the 1950s drew to a close the DK400 cars gave way to the DR450, a lengthened Majestic Major, which was produced in much greater numbers than the DK400. Jaguar converted Daimlers into badge-engineered top-of-the-line Jaguars and the DR450 was replaced by the DS420, a lengthened Mark X platform and engine with a greatly enlarged passenger compartment.
Despite its status as royal transport, the DK400 was as much as 500 kilograms (1,100 lb) lighter than previous postwar Daimler limousines. A number were used by the Queen Mother and others of the UK Royal Family, for which the last-ever Hooper bodies were provided including an open-top landaulette.