SHANGHAI, Feb 13 (Reuters) - Shares in Hong Kong-listed brokerages fell on Monday after state media reported that Guotai Junan International had suspended the opening of accounts by mainland Chinese clients, potentially hitting its business.

A source at Guotai Junan International's Shanghai-based parent said the move followed unwritten guidance from China's securities regulators aimed at discouraging illegal money outflows.

It also came after the China Securities Regulatory Commission (CSRC) banned online brokerages Futu Holding and UP Fintech Holding from soliciting new business from mainland investors on Dec. 30.

Mainland investors are an important source of revenue for Hong Kong-based brokerages, so the ban would have a negative impact on future businesses, said the Guotai Junan source.

Guotai Junan did not immediately reply to Reuters requests for comment.

Guotai Junan's Hong Kong-listed shares fell 0.7%. Guolian Securities and Haitong International lost 3.3% and 2.3%, respectively.

Meanwhile, Hong Kong-based Bright Smart Securities said it would suspend mainland China clients’ accounts starting from Feb. 16 until further regulatory guidance is given, according to a notice sent to clients viewed by Reuters.

"Considering China's regulators said some Hong Kong brokerages conducted illegal cross-border business, we have to wait for regulators to clarify relevant rules," the brokerage firm said in the letter to clients.

Its shares tumbled 12.3% on Monday.

Bright Smart confirmed the suspension of mainland clients' accounts in a filing to the Hong Kong exchange later on Monday, adding "it will not have any impact on the Group’s business and income".

Mainland Chinese can buy overseas securities via official channels such as the cross-border investment scheme QDII, and the Stock Connect programme.

However, many Chinese open stock trading accounts in Hong Kong, potentially bypassing China's strict capital control.

The CSRC has said that Futu and UP Fintech Hong Kong have conducted cross-border securities businesses involving domestic investors without regulatory consent, contravening Chinese laws. (Reporting by Samuel Shen, Jason Xue and Brenda Goh; Editing by Shri Navaratnam, Mark Potter and Emelia Sithole-Matarise)