By Anthony Boadle

BRASILIA, Oct 3 (Reuters) - In a decision that could boost President Dilma Rousseff's chances of re-election next year, Brazil's electoral court ruled on Thursday that her main potential rival failed to gather enough signatures to register her new party in time.

Environmentalist Marina Silva has until Saturday to decide whether to run on the ticket of an existing party to make her second bid for president next October.

The court ruled 6-1 against the creation of her party, called the Sustainability Network, because it fell short of the required 492,000 signatures. Silva blamed electoral notaries across Brazil for failing to validate 95,000 names on time.

After the ruling, Silva said she would announce on Friday whether she would run on another ticket, but added there had been no talks with other parties and she planned to keep building her own party to renew Brazilian politics.

"It's only a matter of time until it gets approved. We might not be registered, but we have the most important thing: ethics. We will become stronger," she told reporters at the court.

Silva, 55, a former senator and environment minister, and the daughter of rubber tappers, won 19.6 million votes to place third in the last election in 2010, won by Rousseff.

Silva risks losing that support if she runs with another party because it could undermine her image as a fresh voice whose lack of ties to traditional Brazilian politics and reputation for honesty have appealed to young Brazilians.

"She will lose her political capital because she will look like just another politician," said Andre César, an analyst with Brasilia-based consultancy Prospectiva Consultoria.

The Superior Electoral Court's decision is good news for Rousseff, who is widely expected to seek a second term, because it could narrow the field and improve her chances of winning outright with no need for a runoff if Silva does not run on another party's ticket.

Recent opinion polls show Rousseff recovering from a dramatic drop in her approval ratings following a wave of massive street protests in June against corruption and poor public services that shook Brazil's political establishment. Opinion polls have confirmed Rousseff as the clear front-runner.

Silva, the only politician to gain ground from the street protests, narrowed Rousseff's lead to 8 points in July, but the president by mid-September had pulled 22 points ahead of her.

The main opposition PSDB is expected to field Senator Aecio Neves, who has failed to gain traction as a Senate leader since 2010 and has fallen to 11 percent of voter intentions.

Eduardo Campos, the governor of Pernambuco state, has thrown his hat in the ring and pulled his party out of Rousseff's coalition government.

But his national voter support is a negligible 4 percent, if elections were held today.

The cast of possible candidates for next year's election shrank earlier this week when Jose Serra, who was considering a third bid for president on another party ticket, announced he was staying in the PSDB.