"We cannot rule out that the proceedings may take a while yet until a final ruling is handed down," Winterkorn told a general assembly of Porsche shareholders in this southwestern German city
"For this reason, there could be a delay with the planned merger. Let me be quite clear about this: the integrated automotive group will happen."
VW had planned to wrap up its takeover by next year, and it was the first time anyone has spoken of a possible delay.
As part of the complex deal that will see Porsche's holding group also own shares in VW, Porsche shareholders will be asked to approve a capital increase worth €5 billion ($6.95 billion) in late November.
Winterkorn warned that a "tax hurdle" might also delay the tie-up.
Porsche faces legal action by around 30 US investment funds which accuse the German car maker of having manipulated the stock market in 2008 and hiding its intentions when it attempted to take over VW.
The value of VW shares spiked above €1,000 at the time, forcing some investment funds to unwind speculative positions in a hurry, and they have sued for around $2 billion in damages and interest.
But the maker of the iconic 911 sports car rejects those charges, and Winterkorn said he considered them "inadmissible and without merit."
"We firmly believe that the facts are on our side and that we have the better case in all of these legal issues," he added.
Porsche took on heavy debt as it tried to buy the much bigger VW but then had to be saved in turn by Europe's biggest automaker, which now owns 49.9 percent of Porsche.
VW expects to complete the takeover and make Porsche the VW group's 10th brand.