SAO PAULO, June 6 (Reuters) - Brazilian soybean and cotton companies on Thursday joined the biofuels and food lobbies to blast new rules for use of tax credits, increasing the odds the measure will be rejected by a Congress heavily influenced by farming interests.

Backlash for the measure represents the latest test in President Luiz Inacio Lula da Silva's shaky relationship with the powerful agribusiness sector, which had supported his far-right predecessor, Jair Bolsonaro.

The new measure on tightening the use of tax credits was included in an executive order sent to Congress on Tuesday. It takes effect immediately but needs Congressional approval within four months to remain valid.

The domestic soybean market saw few deals on Wednesday and business remained slow Thursday while traders and processors assess the impact of the measures, said Luiz Fernando Roque, an analyst at consultancy Safras & Mercado.

Chicago Board of Trade soybeans rose on Thursday as the new rule made traders and producers hopeful U.S. soy export business could get a boost.

Abiove, which represents soybean processors including Bunge and Cargill, claim the move will make them less competitive, penalizing soy farmers and putting investment plans at risk.

According to Abiove, the measure will force down prices paid to soy growers by 4% "and impact the current value of soy by up to 5%."

National soy lobby Aprosoja said it fears receiving less money for crops as the industry risks losing estimated tax credits of 6.5 billion reais ($1.24 billion) from the measure.

Arlan Suderman, chief economist at StoneX, said Brazilian soybean processors and biofuel producers will essentially have higher tax costs and lower margins, adding "that loss of revenue is expected to shift some crush and biofuel activity to Argentina and to the United States, although the scope of that shift is not yet known."

Anec and Anea, which speak for grains and cotton exporters, dubbed the measure "a grave institutional setback" in a joint statement. They called on Congress to reject the rule immediately or else open an ample debate with the companies to discuss its impact.

Anec members alone account for 74% of Brazilian corn and soybean exports worth $66.86 billion. ($1 = 5.2433 reais) (Reporting by Ana Mano in São Paulo; additional reporting by Marcela Ayres in Brasília; Editing by Leslie Adler and David Gregorio)