May 18 (Bloomberg) -- Workers at Porsche SE staged a strike to demand that the sports-car maker stay independent, in a further blow to talks on a planned merger with Volkswagen AG that Europe’s largest carmaker put on hold yesterday.

About 6,000 employees at Porsche’s main Zuffenhausen plant and two other factories near Stuttgart, Germany, joined a rally today addressed by Uwe Hueck, Porsche’s top labor leader. Hueck urged workers to be “self-confident” as Porsche is struggling with 9 billion euros ($12 billion) in net debt.

“Porsche will remain independent, Porsche has to remain Porsche,” Hueck told employees at the carmaker’s research and development center in Weissach, Germany. Volkswagen Supervisory Board Chairman Ferdinand Piech, who said on May 11 that VW won’t help “solve” Porsche’s debt, didn’t show up at a board meeting today that included members of Porsche’s controlling families.

The Porsche and Piech families, who together own half of Stuttgart-based Porsche, agreed on May 6 to create an “integrated” carmaker that would put Porsche alongside VW brands including Skoda and Audi. Talks between the automakers to hash out details of a merger are on hold following Piech’s May 11 remarks and after Bernd Osterloh, Volkswagen’s works council chief, asked to quit the negotiations.

“Piech made a tactical move,” said Frank Schwope, an analyst with NordLB in Hanover, Germany. “His preferred solution seems to be the sale of Porsche’s carmaking business to Volkswagen, and evidently he felt that other proposals would be made at the meeting.”

Shares Fall

Porsche fell 4 cents, or 0.1 percent, to 41.21 euros in Frankfurt. The stock has slumped 28 percent since the May 6 announcement, cutting the carmaker’s market value to 7.26 billion euros. Volkswagen rose 4.74 euros, or 2.2 percent, to 225.05 euros, valuing the company at 71.4 billion euros.

“There is currently no atmosphere for constructive talks,” Christine Ritz, a spokeswoman at Wolfsburg, Germany- based Volkswagen, said yesterday in a telephone interview. In a statement, Porsche said that, while a meeting scheduled for today had been canceled, negotiations with VW will resume.

Speaking to reporters on May 11 in Sardinia, Italy, Piech said that Porsche Chief Executive Officer Wendelin Wiedeking and Chief Financial Officer Holger Haerter were partly responsible for the sports-car maker’s increasing debt.

A spokesman for Piech couldn’t be reached for a comment.

‘War Has Erupted’

“War has erupted again between Volkswagen and Porsche,” said Ferdinand Dudenhoeffer, director of the Center for Automotive Research at the University of Duisburg-Essen. Dudenhoeffer was head of marketing strategy and research at Porsche from 1987 to 1990. “Piech is behind that.”

Porsche, the maker of the 911 sports car, owns about 51 percent of Volkswagen, whose automotive division had 10.7 billion euros in net cash as of March 31. Porsche has been accumulating Volkswagen shares since 2005 to protect ties to its largest supplier.

Porsche Supervisory Board Chairman Wolfgang Porsche was struggling to raise financing to boost the stake to 75 percent and had been at loggerheads with Piech, his cousin, about how to unite the carmakers. The May 6 agreement between the families effectively put on hold Porsche’s plan to further bolster its stake in Volkswagen by acquiring VW shares.

Porsche has no plans to sell some call options linked to Volkswagen shares, Albrecht Bamler, a spokesman at the sports- car manufacturer, said today.

Piech vs Porsches

The Porsche family is upset over Piech’s remarks and is concerned that they may hurt the value of the carmaker, Der Spiegel magazine reported on its Web site. When asked whether Volkswagen would pay 11 billion euros for Porsche AG, the operating unit of Porsche SE, Piech said that the amount is “definitely a few billion too high,” according to the magazine.

Porsche must ensure “absolute transparency” in its talks on combining the carmakers, Volkswagen CEO Martin Winterkorn wrote in a letter to senior managers dated yesterday.

“We must get a clear idea of the true state of affairs at Porsche,” he wrote. “It’s in the interest of all concerned to ensure that there’s no threat to Volkswagen’s financial stability.”

On May 7, a day after the initial agreement, Porsche dropped the most in at least 13 years on the Frankfurt exchange. The stock has fallen 21 percent this year.

Winterkorn Backs Osterloh

Winterkorn said he backed a decision by Osterloh, the chief labor leader at Volkswagen and a member of its supervisory board, to quit the talks with Porsche.

“We will not allow anyone to pressurize us into taking precipitate action,” Winterkorn wrote, adding that negotiations with Porsche require a “constructive” atmosphere.

“This is not the case at the moment,” the CEO wrote. “We’re not in any hurry.”

The 72-year-old Piech is a grandson of Ferdinand Porsche, who founded the sports-car manufacturer and was Volkswagen’s first leader when the carmaker was set up under Adolf Hitler’s Nazi regime in the 1930s. In addition to leading the supervisory board at Volkswagen, where he was CEO for nine years until becoming chairman in 2002, Piech is a member of Porsche’s board.

Any agreement between the two carmakers will require approval by Volkswagen’s home state of Lower Saxony, which has a right to veto decisions through its 20 percent stake in VW. The automakers, worker representatives and Lower Saxony officials will decide on the new group’s structure over a four-week period, Porsche said on May 8.

To contact the reporter on this story: Andreas Cremer in Berlin at acremer@bloomberg.net; Richard Weiss in Weissach, Germany, via rweiss5@bloomberg.net.

Last Updated: May 18, 2009 12:18 EDT