The three bidders for Opel, a European unit of General Motors, include Canadian auto parts maker Magna, Itay's Fiat and RHJ International SA, a fund that has some former holdings of US private-equity firm Ripplewood Holdings LLC.
"We now have three offers for an Opel takeover, but that doesn't mean that one of them will automatically come to fruition," Guttenberg told Bild am Sonntag newspaper.
The minister said he was unconvinced that the three bidders would ensure that bridge loans provided by the government wouldn't be squandered.
"We must first have a high degree of certainty that the significant tax money we will have to provide is not lost," he added. "From my point of view, none of the three offers so far provides this certainty in a sufficient way," he said.
"If these deficits were to remain, an orderly insolvency would clearly be the better solution - it also could open opportunities for the future of Opel," Guttenberg added.
His comments came a day after reports that Fiat had sweetened its offer aimed at winning state aid. But there were no details about how Fiat had improved its offer.
Fiat, which is in talks to form an alliance with Chrysler in the US, raised its offer for Opel after Magna emerged on Friday as the leading bidder.
Russelsheim-based Opel has said it needs €3.3 billion in state aid to survive as GM struggles to avoid a June 1 bankruptcy.
The fate of Opel, an industrial icon dating back to the 19th century and directly employs around 25,000 people in Germany, has become a hot-button political issue with barely four months to go until general elections.
Chancellor Angela Merkel, up for a second term in the September 27 vote, is prepared to pull out all the stops to save Opel from collapse, but writing a blank cheque on behalf of taxpayers could hurt her re-election hopes.