HONG KONG (Reuters) -Global investment firms Carlyle Group and EQT, alongside regional players HongShan Capital Group (HSG) and Boyu Capital, are preparing final offers for a controlling stake in Starbucks' China operations, said five people with knowledge of the matter.

Starbucks has asked them to submit binding bids by early October, said three of the sources, who declined to be identified as the information was private.

An agreement could be reached by the end of next month, one of them added.

Starbucks had invited about 10 potential buyers to submit non-binding bids by early September, with most offering to value the China business at as much as $5 billion, Reuters reported last month.

Starbucks has recently decided to sell control of its China operations to the final buyer, said two of the sources. The size of the stake has not yet been disclosed.

The final round of bidders also includes Chinese private equity firm Primavera Capital, which is likely to team up with one of the four primary bidders, said two of the sources.

The Seattle-based coffee giant is seeking to retain control of its coffee bean roasting facility in the world's second-largest economy, said two of the sources, with one adding that was for quality control purposes.

Terms of the deal structure, including the size of the stake being sold, remain negotiable, said one of the sources.

Starbucks has said that it would maintain a meaningful stake in the China business.

The company did not immediately respond to comment regarding the ongoing sale process, nor did EQT and Boyu.

Carlyle, Primavera and HSG, formerly known as Sequoia China, all declined to comment.

Goldman Sachs, which is advising Starbucks on the sale, declined to comment.

The sale comes as Starbucks faces declining market share in China - home to more than a fifth of its cafes - due to intensifying competition from local rivals.

Its market share fell sharply to 14% last year from 34% in 2019, according to Euromonitor International data.

To counter these challenges, the chain has since implemented measures such as reducing prices for select non-coffee beverages in China and accelerating the introduction of new, localised products.

Comparable-store sales in China increased 2% in the quarter ended on June 29, versus zero growth in the previous quarter.