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 and flooded forests.
Enough mine waste to fill 13,000 Olympic-size swimming pools poured into 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.
She said in a 222-page ruling: "BHP's control of Samarco, their assumption of responsibility for risk assessment, management and control of the tailings dam, and their full participation in the tailings dam operations, gave rise to a legal duty to avoid harm caused by any act or omission that was negligent, imprudent or lacking in skill".
O'Farrell said that BHP had been negligent or imprudent when it failed to carry out studies and remediation recommended by engineers, allowing waste material to become saturated with water and when Samarco continued to raise height of the dam.
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.
The trial began in October 2024, just days before Brazil's federal government reached a multibillion-dollar settlement with the mining companies.
Under the agreement, Samarco — which is also half-owned by Brazilian mining giant Vale — agreed to pay 132 billion reais ($A35 billion) over 20 years.
The payments were meant to compensate for human, environmental and infrastructure damage.
BHP had said the UK legal action was unnecessary because it duplicated matters covered by legal proceedings in Brazil.
BHP spokesman Brandon Craig said in a statement that 240,000 claimants in the London lawsuit "have already been paid compensation in Brazil".
with AP and PA
