Brazil's Congress on Thursday (AEDT) voted to open up its deep-water oil fields to foreign investors by scrapping Petroleo Brasileiro's obligation to be the area's sole operator.

The legislation was approved by a 292-to-101 vote, following a bad-tempered debate during which opponents, many dressed in the orange jumpsuits of the state-controlled oil company, repeatedly attempted to delay proceedings. Various amendments to the legislation will be debated next week before President Michel Temer signs it into law.

In a significant change to Brazil's offshore oil industry regulations, companies other than Petrobras can now operate blocks in the largest deep-water deposits discovered this century. Until now, the heavily indebted state-controlled oil producer was legally obliged to operate all the pre-salt fields with a 30 per cent minimum stake.

Gas flares from a floating production and storage vessel during exploration operations off the coast of Brazil. Petrobras will no longer have sole rights. DADO GALDIERI

Legislation to open up the area to foreign investors began under former President Dilma Rousseff, but many within her left-wing Workers' Party were deeply hostile to the idea. This week's vote, backed by the Petrobras leadership, marks the most investor-friendly change in regulation since the 1997 oil law that ended the state company's monopoly in Brazil.

"It's a very important sign that the government is in control of this process to open up the industry," said Milton Costa, the secretary-general for the Brazilian Petroleum Institute an oil industry lobbying group. "There's no way Petrobras can handle the pre-salt requirements by itself."