By Sam Tobin

LONDON (Reuters) – BHP said on Wednesday that allegations a pursuit of profit over safety contributed to Brazil's worst environmental disaster were "far-fetched and unjustified", as the miner opened its defence to a mammoth lawsuit at London's High Court.

More than 600,000 Brazilians, 46 local governments and around 2,000 businesses are suing BHP over the 2015 collapse of the Mariana dam in southeastern Brazil, which was owned and operated by BHP and Vale's Samarco joint venture.

The dam's collapse unleashed a wave of toxic sludge that killed 19 people, left thousands homeless, flooded forests and polluted the length of the Doce River.

The claimants' lawyers accused BHP of "cynically and doggedly" trying to avoid responsibility as the trial of a lawsuit worth up to 36 billion pounds ($47 billion), one of the largest in English legal history, began on Monday.

They also allege BHP contributed to the collapse of the dam by allowing it to be raised as part of an expansion project, despite an increasing risk of failure.

BHP, the world's biggest miner by market value, is contesting liability and says the London lawsuit duplicates legal proceedings and reparation and repair programmes in Brazil and should be thrown out.

The miner argues it did not own or operate the dam, which held mining waste known as tailings, and that Samarco operated independently. It also says it had no knowledge the dam's stability was compromised before it collapsed.

BHP's lawyer Shaheed Fatima told the court on Wednesday that the case against it was fundamentally flawed.

"The claimants appear to say that BHP was so motivated to make profits from their investment in Samarco that they got behind the wheel, they operated the business, they put profits before safety," she said. "This is unrealistic and illogical."

Fatima added: "The profits before safety allegation, that is particularly far-fetched and unjustified."

She said that BHP's former finance chief Peter Beaven, who is due to give evidence next month, said in a witness statement: "BHP had a culture which was embedded throughout the organisation from top to bottom of safety before anything else."

The ongoing 12-week trial to determine whether BHP is liable to the claimants comes as the Brazilian authorities' negotiations with BHP, Vale and Samarco over a nearly $30 billion compensation deal continue.

Sources close to the negotiations told Reuters a final agreement could have an impact on the London lawsuit, a suggestion the claimants' law firm Pogust Goodhead rejected.

(This story has been corrected to read 'profit over safety,' not 'safety over profit,' in the headline)

(Reporting by Sam Tobin; Editing by Mark Potter)