Anglo-Australian mining giant BHP can be held liable over the 2015 collapse of a dam in southeastern Brazil, London's High Court has ruled, in a lawsuit the claimants' lawyers previously valued at up to $US48.32 billion ($A74.05 billion).
Hundreds of thousands of Brazilians, dozens of local governments and around 2000 businesses sued BHP over the collapse of the Fundao dam in Mariana, southeastern Brazil, which was owned and operated by BHP and Vale's Samarco joint venture.
Brazil's worst environmental disaster unleashed a wave of toxic sludge that killed 19 people, left thousands homeless, flooded forests and polluted the length of the Doce River.
Judge Finola O'Farrell said in her ruling that continuing to raise the height of the dam when it was not safe to do so was the "direct and immediate cause" of the dam's collapse, meaning BHP was liable under Brazilian law.
O'Farrell said Australia-based BHP was responsible despite not owning the dam at the time.
Anglo-Australian BHP owns 50 per cent of Samarco, the Brazilian company that operates the iron ore mine where the tailings dam ruptured on November 5, 2015.
Enough mine waste to fill 13,000 Olympic-size swimming pools poured into the Doce River in southeastern Brazil.
The ruling only addressed liability. A second phase of the trial will determine damages.
The case was filed in Britain because one of BHP's two main legal entities was based in London at the time.
BHP said it would appeal against the ruling and continue to fight the lawsuit.
BHP's president minerals Americas Brandon Craig said in a statement that 240,000 claimants in the London lawsuit "have already been paid compensation in Brazil".
"We believe this will significantly reduce the size and value of claims in the UK group action," he added.
with AP