Judge Joao Gebran Neto said in a three-hour judgement that Lula was one of the architects "of a sophisticated scheme of fraud and corruption" that had weakened Brazil's entire political system.
He voted to increase Lula's sentence from 9.5 years to more than 12 years.
The other two judges were to present their judgements later Wednesday.
The court had convened amid high security in the tense southern city of Porto Alegre to rule on an appeal by the hugely popular leftist icon against his July corruption conviction in Brazil's sprawling "Car Wash" graft scandal.
The stakes are high, and thousands of Lula supporters and opponents gathered in the city to await the ruling.
Confirmation of the sentence would effectively end Lula's hopes of running in this year's presidential election, which he is favored to win.
Lula was not in court. Instead he was hundreds of kilometers (miles) away near Sao Paolo, addressing former colleagues in the powerful metalworkers' union he once led.
"I am extremely calm, with the awareness that I have committed no crime," he told union members.
- Court review -
Lula is accused of being gifted a three-floor seaside apartment from Brazil's OAS construction group in exchange for public contracts from state-controlled oil company Petrobras during his 2003-2010 presidency.
Defense lawyer Cristiano Zanin Martins had told the court it was clear OAS owned the apartment, and that Lula "never got the keys and never spent a night there."
But in his judgement, Gebran Neto said there was evidence "that the triplex apartment, from the beginning, even before the OAS took over the works, was reserved for President Lula."
The defense had argued that no title existed in his name, but the judge said the lack of a document was precisely to hide the real recipient of the apartment.
Lula addressed thousands of cheering supporters at a rally in Porto Alegre on the eve of the court sitting, many wearing the red T-shirts of his Workers' Party (PT).
He was joined on the podium by his handpicked successor Dilma Rousseff, who was impeached in 2016 for breaking budget rules.
Lula, Rousseff and Brazil's leftist leaders say they are the victims of a plot by the judicial, political and business sectors to sideline the PT.
Rousseff had earlier denounced what she called the "third stage of the coup" against Lula.
The first, she argued, was her impeachment, and the second was the austerity budget introduced by her successor Michel Temer. The third is "aiming to destroy the PT and especially our leader."
Observers believe the court is likely to uphold the conviction -- effectively scuppering Lula's chances of running in the October presidential election. Lula could appeal, but the process could drag on for months.
"I consider that the degree of guilt is extremely hight," the judge said. He added that the sentence would only be enforced once the appeals process had been exhausted, ruling out the possibility of Lula going directly to jail.
The Eurasia Group consultancy said the court will probably uphold the conviction "and Lula will probably be disqualified in final decisions by the Supreme Court and the electoral court" but closer to election time.
- High security -
Authorities in Porto Alegre have mounted a special security operation to keep opponents and supporters of Lula apart over fears of clashes.
Helicopters hovered near the courthouse, and even ships were being used to protect the building, which is located next to a river.
Argentine ex-footballer Diego Maradona used social media to voice his support for Lula, and released pictures of him holding a Brazil football jersey printed with Lula's name.
"My dear Lula, Diego is with You," wrote Maradona in Portuguese.
But Lula is also hated by many Brazilians who want to see him behind bars. Similarly, the markets are betting that the courts will confirm his sentence and block his way to the presidency. Sao Paolo's stock exchange was up by more than 2.0 percent at midday.
"Markets cannot hide their enthusiasm: a ruling against Lula, who has promised to revert part of President Michel Temer's business-friendly reforms, is widely seen as a fatal blow to his presidential bid," said Silvio Cascione in a note from the Eurasia consultancy.
Whatever the verdict, the court decision will constitute a new test for Brazilian democracy.
"If Lula can't run, the election is very uncertain and we would have five or six candidates with the possibility of reaching the second round, which would make the 2018 elections the most unpredictable since the restoration of democracy (in 1985)," political scientist Mauricio Santoro of Rio de Janeiro State University told.