Investor demand for General Motors stock has been so strong that the company will expand its initial public offering by 31 percent, to 478 million common shares, a person briefed on the sale said Tuesday.

The move could make GM's IPO the largest in history for a U.S.-based company. If GM's sale of preferred shares is included, the offering could have a total value of $21.6 billion, topping Visa Inc.'s $19.7 billion IPO in 2008, according to the IPO tracking firm Dealogic.

GM is expected to announce the final price of the IPO on Wednesday and shares will start trading the following day at a price of $33 each, according to the person, who asked not to be identified because he is not authorized to speak publicly about the sale.

Most of the additional shares will be sold by the U.S. government, said the person. A union health care trust would sell a small part of the additional shares, the person said.

Bankers handling the GM will take an option to sell another 55 million shares, bringing the total value of the common shares sold to $17.6 billion. The company will sell preferred shares worth $4 billion, bringing the total value of the deal to $21.6 billion.