Australia's wheat exporter AWB has been suspended from US government contracts and faces permanent exclusion, for paying bribes to Iraq's former regime.
AWB was the largest single supplier of humanitarian goods to Iraq
The step was taken "based on evidence of illicit activities", said US Agriculture Secretary Mike Johanns.
Last month, a judicial inquiry found AWB broke UN oil-for-food programme rules by paying Saddam Hussein $222m (£112m) to secure contracts.
It also recommended that 11 former AWB executives face corruption charges.
The high-profile commission inquiry, chaired by former judge Terence Cole, cleared the Australian government of involvement in bribes.
The Australian Wheat Board had been the largest single supplier of humanitarian goods to Iraq under the UN programme, which ran from 1996 to 2003.
The wheat supply came within a UN programme aimed at allowing Iraq to use money from oil exports to purchase food and medicine, to counter suffering caused by international sanctions under Saddam Hussein's regime.
'Bribery and kickbacks'
The US agriculture secretary said on Wednesday that the Foreign Agricultural Service was taking action against AWB and its affiliates, as well as 11 individuals and a Minnesota-based company.
The order was based on evidence of "bribery, kickbacks and similar behaviour resulting in payments to the Saddam Hussein regime," Mr Johanns said.
He said the illegal activities "were made more egregious by attempts to cover up their actions".
The order is effective immediately and prohibits participation "in any US government procurements as well as many other US government programmes such as loan guarantees".
"We have a duty to protect the public interest by ensuring the firms and individuals with whom we do business abide by the law," Mr Johanns said.
Two weeks ago, Australia's senate rushed through a bill temporarily stripping AWB of its 67-year-old monopoly powers to control the export of wheat from the country.
AWB's power to veto export applications from other Australian grain exporters was transferred to Agriculture Minister Peter McGauran for six months.