SHANGHAI, Jan 4 (Reuters) - China Mobile Ltd will buy back up to $12.6 billion worth of its Hong Kong listed shares on the market, as the company prepares for its 48.7 billion yuan ($7.7 billion) listing in Shanghai on Wednesday, China's biggest public share offering in a decade.

The company told the Hong Kong Stock Exchange it would press ahead with a plan to buy back up to 2.05 billion shares using existing cash and working capital.

China Mobile's Hong Kong shares rose 1.91% on Tuesday to close at HK$48 ($6.16), the highest since early November, before the news was announced.

In the stock exchange announcement, the company said the buyback would represent about 10% of its issued shares in Hong Kong and the purchased shares would be canceled.

At the current Hong Kong price, the deal would be worth about $12.6 billion, but would rise if the stock gained ground during the buyback period.

The world's largest mobile network operator by subscribers, China Mobile sold 845.7 million shares at 57.58 yuan ($9.06) each in Shanghai, the company said in an exchange filing on Tuesday, announcing the debut date.

At Tuesday's close, the company's Hong Kong-listed shares traded at a 32% discount to the stock's Shanghai offering price.

The size of China Mobile's share sale would be expanded to 56 billion yuan if an over-allotment option is fully exercised, making the public offering China's fifth-biggest on record, according to Refinitiv data.

China Mobile's smaller state-owned rivals, China Telecom and China Unicom, are already listed in mainland China.

The three were delisted from the New York stock exchange after a Trump-era decision to restrict investment in Chinese technology firms, amid continuing tensions between Washington and Beijing.

($1 = 6.3521 yuan)

($1 = 7.7981 Hong Kong dollars) (Reporting by Samuel Shen and Scott Murdoch Editing by Stephen Coates and Mark Potter)