Marina Silva has not yet declared whether she will be a candidate
Brazil's ex-environment minister Marina Silva is leaving the governing Workers Party, paving the way for a possible presidential run in the 2010 election.
Ms Silva, a staunch defender of the Amazon rainforest, said she was likely to join Brazil's Green Party.
She left the government in May 2008, citing difficulties in pursuing an environmental agenda.
If she runs, Ms Silva is very unlikely to win, analysts say, but her candidacy could have some impact on the result.
President Luiz Inacio Lula da Silva enjoys high popularity ratings but under the Brazilian constitution he cannot run for a third consecutive term.
He has repeatedly rebuffed any talk of attempting to change the constitution.
His favoured successor is his chief of staff, Dilma Roussef.
The 2010 race is likely to be between her and Sao Paulo state governor Jose Serra of the PSDB party.
Marina Silva, who said she had still not decided on a presidential run, was vocal during her time in office in blaming the deforestation of the Amazon on Brazilian cattle ranchers and farmers.
She unsuccessfully opposed several government infrastructure projects in the Amazon rainforest, including two big hydroelectric dams on the River Madeira, and a major new road.
The child of rubber-tappers from the Amazonian state of Acre, Ms Silva was illiterate until the age of 14.
She worked with the rainforest activist Chico Mendes, who was murdered in 1988, and she was appointed environment minister when Lula won the presidency in 2002.