The program, unveiled in May to halt a truckers' strike over rising diesel prices, set prices too low and has yet to pay out compensation to most diesel importers, FCStone said in a statement.

"Private imports are paralyzed, and Petrobras is practically the only agent bringing the product to Brazil," FCStone said.

That means most companies have stopped selling the fuel, forcing Petrobras to supply most of the market at a loss and raising the specter of a shortage in the coming months, the commodity broker added.

"Unless oil regulator ANP raises diesel prices and starts paying subsidies, we will have diesel shortages in the last months of the year or Petrobras will have to face the cost alone of supplying the whole market at a loss," FCStone said.

The subsidy plan ended the strike, which had paralyzed Latin America's second largest economy from late May to early June. But it raised the specter of a new and costly round of state meddling in the affairs of Petrobras, still the world's most indebted oil company.

However, Petrobras Chief Financial Officer Rafael Grisolia told Reuters on Friday the firm expected to receive 2 billion reais to 2.5 billion reais ($484.04 million to $605.05 million) from ANP within two weeks to compensate for subsidies.

Brazil will need to import some 5.5 million cubic meters of diesel in the last four months of the year, a 12 percent increase over the same period last year, FCstone said.

The growing need comes after an explosion in August at Replan, Petrobras' largest refinery, hurt output and as a thriving agriculture industry drives demand higher.