May 18 (Reuters) - Brazil's Federal Audit Court (TCU) on Wednesday voted to approve the privatization of the country's largest utility, state-run power company Eletrobras, by diluting its controlling stake through a share issue.

The approval meant the stock offer can be made as early as next month, a source close to the process said.

The timeline of the offer should be defined by early next week and the offering is expected to happen by June 15, said the source, who spoke on condition of anonymity.

Shortly after the decision, Brazil's Mines and Energy Ministry welcomed the approval, adding it "is a historic day for Brazil."

The green light came despite opposition by one of the court's ministers, the reviewer of the process, Vital do Rego. He was the only who voted against the approval, while seven others ministers backed the privatization.

According to a preliminary assessment by the government, the offer could raise around 25 billion reais ($5.03 billion).

Eletrobras will still analyze the entire ruling to evaluate if the court's deliberations impose adjustments to the terms that had been approved by the government and its shareholders, the company's chief executive, Rodrigo Limp, said on Tuesday.

($1 = 4.9685 reais) (Reporting by Letícia Fucuchima; Aditional reporting by Rodrigo Viga; Writing by Peter Frontini; Editing by Richard Pullin and David Gregorio)