Elliott commissioned a report by FTI Consulting to "move the issue forward" and has called on BHP to commit to a full, independent review of unification by February 20 when the company presents its half-yearly results.

The report said the change would cost $391 million, provide valuable strategic optionality and would preserve BHP's ability to use PLC's carry-forward tax loss assets following unification.

Elliott targeted BHP with a "Value Unlock" plan starting last April, asking for unification, demerging its US petroleum assets and adopting a policy of consistent capital returns to shareholders - proposals BHP said it reviewed and concluded the costs and risks would "significantly outweigh the potential benefits".

Elliott said BHP had taken a number of positive steps recently, including electing Ken MacKenzie as chairman and committing to exit US onshore assets, but said "more significant opportunities remain for BHP to maximise value and returns for its shareholders, with unification topping the list".

It said BHP's preliminary analysis in April last year was "woefully inadequate" in assuring shareholders the costs and benefits had been comprehensively and independently explored.