Porsche said in a statement it had boosted its stake in Volkswagen to 35.14 percent from 31 percent, giving it a "permanent majority at the VW general assembly."
The move makes VW an "effective" Porsche unit under German share owner and takeover law.
"The goal remains to increase our stake in Volkswagen to more than 50 percent," Porsche chief Wendelin Wiedeking said. "Today's step is a further milestone along the way."
Despite recent tensions between the management of the two German automaking legends, Porsche said it looked "forward to the continuation and deepening of the trusting cooperation with the Volkswagen board." It added that it hoped for a quick resolution of the conflict between the works councils of Porsche and VW.
Volkswagen's works committee is wary of the takeover and of Porsche boss Wiedeking for crossing swords with the trade unions, which are extremely powerful at VW.
Porsche has said since early March that it planned to take over the much bigger VW, and on Tuesday that it planned to increase its stake in VW again in the coming months, pending approval from regulators.
The company is also seeking to counter efforts by the German government and the state of Lower Saxony, where VW is based, to retain a blocking minority held by the state, which owns around 20 percent of the shares in VW.