Paulo Fridman/Bloomberg

Carrefour, the giant French retailer, said on Tuesday that it had received an offer to merge its Brazilian operations with those of Companhia Brasileira de Distribuição, owned by Grupo Pão de Açúcar.

The joint venture, proposed by the investment firm Gama, would create the largest retail company in Brazil, with total combined sales estimated at 30 billion euros ($43 billion) this year. But the deal is being contested by Carrefour’s French rival, Casino, which owns a large stake in Grupo Pão de Açúcar.

To finance the deal, Gama has said it has commitments for 2.5 billion euros in new capital and debt from its parent, BTG Pactual, and the Brazilian National Development Bank.

Gama and Carrefour would split the ownership of the joint venture 50-50, and the investment firm would take an 11.7 percent interest in the French retailer through an issuance of new preferred shares. Gama would also purchase up to 6 percent of Carrefour shares on the open market, making it the company’s largest shareholder.

Gama said it would strike a shareholder agreement with other big Carrefour investors — Blue Capital, Colony Blue Investor and Groupe Arnault — which together control 14 percent of Carrefour’s shares and more than 20 percent of the voting rights.

On Tuesday morning, Carrefour’s shares rose 78 cents, or 2.93 percent, to 27.23 euros in trading in Paris. In September, the shares were trading at more than 41 euros, but have since slumped as growth has stagnated in its home markets.

Gama said the combined company would have 2,386 stores in Brazil, and it estimated annual cost savings of 700 million euros from the deal.

One potential obstacle is Carrefour’s rival Casino, which has a 37 percent stake in Grupo Pão de Açúcar. Casino has taken the president of Grupo Pão de Açúcar, Abilio Diniz, to court for not disclosing talks with Carrefour.

The retailerreacted harshly to the Carrefour announcement on Tuesday, saying “it is not a spontaneous proposal from Gama, a financial investment vehicle, but a long-standing illegal planned financial transaction between Carrefour and Abilio Diniz.”

Casino said it “has the authority to oppose this project according to the existing agreements,” and that it would “examine how to best defend the corporate interests of C.B.D. and its shareholders” from a transaction of an “obvious hostile nature.”

Casino shares slid sharply on Tuesday, falling 2.73 euros, or 4.14 percent, to 63.17 euros in Paris trading.