GENERAL Motors Co's initial public offering has become the world's biggest at US$23.1 billion after underwriters swiftly took up additional shares.

The added shares vaulted GM past the Agricultural Bank of China's US$22.1 billion IPO in July and underscored strong demand for the taxpayer-rescued auto maker's stock.

GM said underwriters led by Morgan Stanley, JPMorgan Chase & Co, Bank of America Merrill Lynch and Citigroup Inc, exercised their full option on an additional 71.7 million common shares worth US$2.37 billion.

They also exercised an option to purchase 13 million preferred shares for US$650 million.

Underwriters had 30 days from the IPO to exercise the options.