The cause of the split was ostensibly the same in both ports: the rebels claimed that the NSFU was reckless and extravagant with its funds, that members had little control over the direction of the organisation, and that its leading officials were unaccountable.
It accused the BSU of sectionalism, and of being the product of the political ambitions of its leading officials, Tommy Lewis and Manny Shinwell.
Unlike the NSFU, the BSU was not granted a role on the National Maritime Board which was formed in 1917 to decide on working practices in the merchant shipping industry.
In May 1921, the Board imposed wage reductions which were resisted by both the BSU and the National Union of Ship's Stewards.
Later that year, these two organisations were merged to form the Amalgamated Marine Workers' Union, (AMWU).