The house flag of the Belfast Mersey and Manchester Steam Ship Co. Ltd, Belfast. A red rectangular flag with a white diamond in the centre and black letters 'BMM'. 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.

The company traded under the name of Belfast Mersey and Manchester Steam Ship Co. Ltd from 1929, specializing in carrying cargo, particularly cattle between Belfast and Liverpool. The company was taken over by Coast Lines in 1945 from its managing owners Samuel Lawther & Sons of Belfast and John J. Mack & Sons Ltd, Liverpool. Its fleet merged with that of the Belfast Steamship Co. in 1960, its vessels retaining their former owner's house flag and funnel colours until the last was withdrawn from service in 1969. The company is now part of P&O.