Ant, backed by Chinese e-commerce giant Alibaba plans to do a simultaneous listing in Hong Kong and Shanghai, in what sources have said could be the world's largest initial public offering (IPO) and come as soon as October.

CITIC, which has in actively pitched for a role in Ant's listing on Shanghai's Nasdaq-style STAR Market, is set to become a joint underwriter for the onshore leg, said three of the people.

The Chinese brokerage's possible role in the Ant IPO comes after it dropped out of the Shanghai flotation of Ant's local rival JD Digits, financial services affiliate of e-commerce company JD.com, said two of the sources.

Many Chinese firms restrict top investment banks from working for competitors until their transactions close due to concerns the banks working on a concurrent IPO timetable may divulge information to the benefit of their rivals.

CITIC did not immediately respond to a request for comment. Ant declined to comment.

All the sources declined to be named as the information was private.

JD Digits said in a filing on Thursday that the company and CITIC had terminated their so-called counseling agreement signed in late June for an IPO on the year-old STAR Market in Shanghai.

Compared to Ant's mega IPO, JD Digits plans to raise between 20 billion yuan and 30 billion yuan ($2.9 billion-$4.4 billion), said two of the people. JD.com did not immediately respond to a request for comment.

China International Capital Corp (CICC) and China Securities Co are the sponsors, the most senior role in the transaction, for the Shanghai tranche of Ant's IPO, according to the firm's exchange filing.

Ant plans to raise more funds on the STAR Market than in Hong Kong, Reuters reported on Wednesday.

CITIC has topped the equity capital markets rankings for mainland China and Hong Kong, with $14 billion worth of deals it and its Hong Kong-based unit CLSA worked on, in the first eight months of the year, according to Refinitiv.

By Julie Zhu and Samuel Shen