Brazil's governing Workers' Party says it will sue a magazine for reporting that Cuba helped to finance the 2002 electoral campaign of the president.
Lula is a friend of the Cuban president
The leading weekly Veja said President Luiz Inacio Lula da Silva's campaign received up to $3m through a diplomat.
Opposition figures said they would call for an investigation. The Cuban and Brazilian governments deny the report.
The row comes as Lula faces a major scandal over bribery, which is alleged to have taken place in Congress.
The president has not been linked to the congressional corruption scandal.
Lula won the election in October 2002 and is an old friend of Cuban President Fidel Castro.
The report in the news magazine said Lula's electoral committee received the campaign funds that September and October, through a former diplomat identified as Sergio Cervantes.
The alleged donation would be a violation of Brazil's electoral law.
Senior opposition figures said the allegations were very serious and needed to be investigated.
"Lula and the PT have surpassed all limits," Congressman Jose Carlos Aleluia of the Liberal Front Party (PFL) was quoted by Reuters as saying on the party website.
Some revived suggestions that President Lula might have to be impeached.
But the PT (Workers Party) called the report part of a plot by Brazil's elites.
"The magazine is a pamphlet for the PSDB (Social Democrats) and the PFL," PT President Ricardo Berzoini was quoted as saying by Reuters.
The magazine did not provide evidence and issued the report based on interviews with former aides of the finance minister, the Associated Press reports.