A Californian jury has ordered chemical giant Monsanto to pay $US2 billion ($2.8b) to a couple who say they contracted cancer after using the commonly used weedkiller, Roundup.

Key points: Alberta and Alva Pilliod used Roundup for more than 30 years, and were both later diagnosed with cancer

Alberta and Alva Pilliod used Roundup for more than 30 years, and were both later diagnosed with cancer Lawyers say there will be more cases against Monsanto

Lawyers say there will be more cases against Monsanto Monsanto's owner, Bayer, says the decision is "excessive and unjustifiable" and it will appeal

It is the biggest payout against the company Monsanto over the commonly used weed-killer Roundup and the third case they have lost.

Earlier this year, a United States jury found glyphosate-based weedkiller Roundup was a "substantial factor" in causing a man's cancer, the second such court ruling against the herbicide's makers in less than a year.

The plaintiffs, Alberta and Alva Pilliod, used Roundup for more than 30 years, and were both later diagnosed with non-Hodgkin's lymphoma.

In a press conference after the verdict Mrs Pilliod thanked the jury.

"We wish that Monsanto had warned us ahead of time about the dangers of using Monsanto [weedkiller]," she said.

"And that there was something on the front of their label that said 'danger may cause cancer'."

Alva and Alberta Pilliod have been awarded $2 billion in a case against chemical giant Monsanto. ( Supplied: Baum, Hedlund Law )

'We are coming after them'

The couple's lawyer Brent Wiessner said there would be more cases against the company.

"This is not the end, this is the beginning, there are tens of thousands of people out there, probably 30,000, or 40,000 people out there," he said.

"And I've told Bayer and Monsanto we are coming after them."

German company Bayer bought Monsanto in a $62.5 billion deal last year and is now responsible for the pay-outs against them.

Bayer to appeal

In a statement Bayer said it was disappointed with the verdict and would appeal. A spokesman called the jury's decision "excessive and unjustifiable".

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President of the National Farmers Federation Fiona Simson said glyphosate had been through a rigorous approval process in Australia.

"In Australia, the registration process of all pesticides involves years of data collection and comprehensive assessment before approvals are granted and a product can be sold on the Australian market," she said.

"Glyphosate is vital to modern day farming systems. Without it we couldn't farm as environmentally sustainably as we do now."

Australia's regulator the Australian Pesticides and Veterinary Medicines Authority (APVMA) would not comment on the latest verdict.

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