The underwriters of Alibaba Group Holding Ltd's IPO have issued additional shares, according to the Wall Street Journal, bringing the IPO's size to about $25 billion and making it the largest initial public offering in history.

The move to exercise what is known as the "green shoe" option as part of Alibaba's initial public offering was widely expected if its shares performed well in the public market.

Last Friday, Alibaba shares soared 38 percent in its first session, as investors clamored for a piece of the company, which is a massive bet on China's growing middle class. If the underwriters issue the maximum amount of 48 million additional shares at the offer price of $68 per share, this increases the amount initially raised last week from $21.8 billion to $25 billion.

The report in the Wall Street Journal online edition on Sunday, which cited unnamed sources, did not specify how many additional shares were issued.

Alibaba declined to comment.

Reuters reported on September 19 that underwriters would decide on whether to exercise the option over the next week or two, depending on how the shares traded.

Underwriters on the Alibaba deal also include Morgan Stanley and JPMorgan Chase & Co, with Rothschild, which does not have underwriting operations, advising Alibaba on the deal.

At its closing share price on Friday, Alibaba has a market value of $231 billion, exceeding the combined market capitalizations of Amazon and eBay, the two leading US e-commerce companies.

Alibaba jumps more than 40% in trading debut	Alibaba makes its mark on Wall Street