The house flag of the Union Castle Mail Steamship Co. Ltd, London. A dark blue rectangular flag with a white diamond in the centre and a white-bordered red saltire. The flag is made of a wool and synthetic fibre bunting. It has a cotton hoist and is machine sewn. A rope and toggle is attached.

In 1853 Arthur Anderson of P&O formed a steamship company for the specific purpose of transporting coal from South Wales to Southampton for use by his own vessels. This was named The Union Steam Collier Company Ltd, after the first ship, 'Union', which only made two voyages as a collier before being requisitioned for use in the Crimean War. When peace came the fleet was put out to charter as The Union Steam Ship Company Ltd, and after failed attempts at trading to Rio and Hamburg, it won the tender for the mail service to the Cape Colony in South Africa in 1857.

The ships carried mails, cargo, and passengers on a service that by contract had to be no more than 42 days. The company routes extended to Natal, Mauritius, Ceylon, and Zanzibar. It enjoyed a monopoly on the South African service until George Currie's Castle Mail Packets Company secured a half share in the mail contract in 1876. The two companies remained rivals until in 1899 the Cape Government announced the mail contract would be given as a 'whole', the non-amalgamation clause was finally dropped, and the two companies merged into The Union-Castle Mail Steamship Company Ltd.

In 1912 the Royal Mail Steam Packet Company bought the entire share capital following on from disputes between the company and the Government. After 19 years, the Royal Mail Group collapsed and Union-Castle regained its independence under the control of Sir Francis Vernon Thompson. The fleet was extensively modernized and by 1938 the mail service to the Cape took only 14 days. In 1956 the company merged with The Clan Line Steamers Ltd to form the British and Commonwealth Shipping Company, which continued to serve the mail route to the Cape, cutting the time to eleven days by 1966. Commercial considerations ended the viability of the mail service which stopped in 1977. Union-Castle operated for a few more years under Universal Reefers, but by 1986 it ceased trading.