NEW YORK, (CNN/Money) - The bankruptcy by auto parts maker Delphi Corp. could mean an $11 billion hit to its former parent General Motors Corp., according to the automaker, and thousands of job cuts at Delphi's North American parts plants.

GM (Research) issued a statement late Saturday estimating its liabilities from the Delphi (Research) bankruptcy filing at as much as $11 billion, due to contract obligations to United Auto Workers members at its former parts unit. That's because the bankruptcy came less than eight years after GM spun off Delphi in 1999.

Still, GM spokeswoman Toni Simonetti told the Detroit News Sunday that the $11 billion figure was a "worst-case scenario."

But GM said it could also see some advantages, as it estimated it is paying Delphi, its largest supplier of parts, a premium of $2 billion annually for those parts above market prices.

"GM believes that a restructuring of Delphi through the Chapter 11 process provides it with an opportunity to reduce or eliminate that purchase price premium, over time, as well as improve the quality of systems, components and parts it procures," GM said in its statement.

Even though the bankruptcy filing was long threatened by Delphi management, the news of the action, and the estimates from GM about liability, sent shares of GM down 3.5 percent in Frankfurt trading early Monday.

Meanwhile, Delphi CEO Robert "Steve" Miller told the Wall Street Journal that Delphi's troubles would require the company to divest, consolidate or close "a substantial segment" of its 45 manufacturing sites in the U.S. and Canada, which employ 49,000 workers.

But he also told the paper he was not sure if the company would ask the U.S. government to take over its pension obligations.

"We have not decided what we will do" with the Delphi pension plan, Miller told the paper.

"We want to try and create a company that accommodates our retirees without having (the pension plan) terminated and turned over" to the government's Pension Benefit Guaranty Corp., he said. The company plan is underfunded by as much as $4.3 billion.

Miller, a 63-year-old turnaround specialist, led Bethlehem Steel Corp. through bankruptcy and handed that company's pension plan to the PBGC, the paper reported.

The filing Saturday came after Miller failed to negotiate a multibillion-dollar bailout plan with GM, Delphi's largest customer, and major wage-and-benefit concessions from the United Auto Workers union.

He said he also plans to renegotiate the contracts and retirement plans of Delphi's 33,000 union workers and 12,000 retirees, the paper reported.

In its filing, Troy, Mich.-based Delphi did not detail which or how many U.S. plants it hopes to get rid of. The company previously identified 11 U.S. plants that were unprofitable and could be closed or sold. (Full story).

The filing in bankruptcy court is the latest sign that the U.S. auto industry's unionized workers face the painful restructuring efforts forced on airline and steel workers amid bankruptcies in those industries, the Journal reported.

The filing also could disrupt the flow of auto parts, force major concessions on the UAW and load more costs on cash-strapped auto makers.

Analysts estimated GM's obligation for retiree benefits at somewhere between $1.6 billion and $6.6 billion. The company's tab for retiree pensions could run another $3 billion to $4.5 billion if Delphi terminates its plan, the Journal said.

UAW President Ron Gettelfinger in a statement called the filing a "bitter pill" and criticized Delphi for sweetening the severance packages of 21 top executives the day before seeking bankruptcy protection.

Delphi announced this past weekend that it is moving John Sheehan from chief operating officer to chief restructuring officer. Robert Dellinger will become chief financial officer, effective immediately.

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