DETROIT - General Motors Co. has asked banks underwriting its initial public stock offering to set aside 5 percent of common stock shares for sale to more than 600,000 U.S. and Canadian workers, retirees and dealers, according to a Securities and Exchange Commission filing Thursday. These potential shareholders will have an opportunity to purchase shares directly during the IPO, an offering typically reserved for people with brokerage firms.

GM notified workers, retirees and dealers this week that they must preregister with the company's direct share program to be eligible to buy IPO shares. Each must buy at least $1,000 in GM stock.

In its SEC filing Thursday, the Detroit-based automaker set the percentage — 5 percent — allocated to this group.

Buyers must register by mail by Tuesday, or online by Oct. 22. Those who don't register won't be able to buy shares at the guaranteed IPO price.

GM has about 79,000 U.S. and Canadian employees and 517,000 retirees and surviving spouses. The Detroit automaker will have 4,500 U.S. dealers at the end of 2010.

The automaker also disclosed Thursday that a conflict of interest exists between Citigroup Inc. and the U.S. Treasury Department, which owns a 61 percent stake in GM, as well as 12.4 percent in common shares of Citigroup. The conflict, while not unusual on Wall Street when investment banks are tapped to underwrite IPOs, requires Citigroup to ask its clients first before making sales to their discretionary accounts.

Such conflicts of interest aren't likely to bar Citigroup from participating in the IPO as long as GM has another independent underwriter participating, said Matt Therian, a research analyst at Renaissance Capital, which tracks IPOs.