The house flag of Port Line Ltd formerly Commonwealth and Dominion Line, London. A rectangular white flag with a blue saltire overall under a red cross. The flag is made of a wool and synthetic fibre bunting. It has a cotton hoist and is machine sewn. A rope and toggle are attached.

The Commonwealth and Dominion Line was incorporated in 1914 using the house flag of the Tyser company. The company had 23 ships that were contributed by J.P Corry & Co, WM Milburn & Co. Thos B Royden & Co and Tyser & Co. The company operated services to Australia and New Zealand. In 1916 the share capital of the company was purchased by Cunard and after the war their ship funnels were painted in Cunard colours. Apart from this the company operated independently from the parent company. In November 1937 the official name of the company became Port Line Ltd. The company became a fourth equal with Blue Star Line, New Zealand Shipping Company and Shaw Savill & Albion Lines in Crusader Shipping Ltd to trade between New Zealand, the Far East and the Pacific Coast of North America.

The Port Line joined a consortium named Associated Container Transportation Ltd in 1966 and this gradually started the run down and replacement of the M.A.N.Z. Line, which ended in 1969. In 1972 the parent company Cunard was taken over by Trafalgar House Investments Ltd. By 1978 Port Line had become an Integral part of Cunard group and in 1982 the last two Port Line ships were transferred to Brocklebank Line.