NEW YORK (Reuters) - General Motors Co has completed the paperwork for an initial public offering, and timing of its filing with the U.S. securities regulators rests with the board of the top U.S. automaker, sources familiar with the process said on Monday.

The initial prospectus, expected to be for $100 million, is likely to be filed with the U.S. Securities and Exchange Commission on Tuesday, two people said, asking not to be named because the preparations for the IPO are private.

GM updated its several-hundred page S-1 document to add a management risk factor after Chief Executive Ed Whitacre said on Thursday he would step down and be succeeded by Dan Akerson effective in September, the source said.

The surprise announcement last week was a factor delaying GM’s IPO filing by several days, several people close to the process have said.

GM’s IPO is expected to raise between $15 billion to $20 billion in one of the largest IPOs ever, the source said.

A group of 10 banks that committed to be the major group of creditors in GM’s $5 billion credit line will also serve as underwriters for the stock offering, sources said.

The underwriting syndicate for GM’s IPO will not be finalized until closer to when the deal prices, but will include minority firms, one source said.

The banks that have agreed to provide the credit line include Bank of America Merrill Lynch, Barclays Plc, Citigroup Inc, Credit Suisse, Deutsche Bank AG, Goldman Sachs, JPMorgan Chase, Morgan Stanley, UBS AG and Royal Bank of Canada, several sources said.

The source said that GM will add more banks to the credit line while keeping the amount unchanged at $5 billion, reducing banks’ commitments from $500 million each.

The full syndicate for the credit line is expected to be finalized by the end of September, one source said.

CHALLENGES AHEAD

An IPO filing would be a step toward independence for GM and a way for the U.S. automaker to begin shedding its “Government Motors” nickname. A successful offering would also be a major coup for the Obama administration, which owns a 61 percent stake in the company after a $50 billion bailout last year.

As the offering nears, U.S. officials and GM executives are facing delicate decisions over potential investors.

Chevrolet cars are seen at a GM dealership in Miami, Florida August 12, 2010. REUTERS/Carlos Barria

GM is considering selling a part of the carmaker’s stock to institutions that would commit to buy and hold major stakes, people familiar with the discussions said.

Such “cornerstone” investors could help attract additional investors by showing they are willing to stake large sums on the IPO.

But the plan could also spark political criticism if foreign investors or union-linked pension funds appear to be getting too good of a deal.

Obama appointee Ron Bloom, who oversees the U.S. government’s investment in GM and Chrysler, and Herb Allison, the former Fannie Mae chief executive officer who now oversees the Troubled Asset Relief Program, have been consulted on the question of how to balance access to the offering by retail investors against the potentially competing goal of maximizing returns for U.S. taxpayers, a source previously told Reuters.

There are also challenges with GM’s management.

A quarterly filing by GM with the SEC on Monday listed the appointment of Akerson, 61, a veteran financial executive with experience in the telecommunications industry, and the appointment of former Microsoft executive Chris Liddell as chief financial officer as risks for the company’s operations.

“Within the past year we have substantially changed our executive management team,” GM said in the filing on Monday in language expected to be echoed in its IPO registration.

“We have elected a new chief executive officer ... and a new chief financial officer ... both of whom have no outside automotive industry experience.”