German sports car maker Porsche has exercised an option to take an extra 3.6% voting stake in Volkswagen, Porsche said on Monday, taking its holding to 30.9% as of Wednesday.
Raising its stake past 30% requires Porsche to make a mandatory takeover offer to other VW shareholders under German law.
Porsche said on Saturday it expected to offer 100.92 euros per VW ordinary share plus the legal minimum for VW preference shares.
"Porsche is firmly convinced that a closer bond with VW through an increase of the stake to more than 30% of the Volkswagen ordinary shares will produce benefits for both partners without diluting or indeed endangering the identity of Porsche," it said in a statement at the weekend.
"We don't want a majority," a Porsche spokesman had said on Saturday.
The announcement caps months of speculation that a bid was imminent, driving VW stock up by more than a third this year to record highs, as many investors believed Porsche part-owner and VW Chairman Ferdinand Piech wanted to cement his control over Volkswagen.
Volkswagen's second-largest shareholder, the German state of Lower Saxony, said on Saturday it would remain invested in the company.