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

A source at Guotai Junan International's Shanghai-based parent said the move followed unwritten "window 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 December 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 a Reuters request for comment.

Guotai Junan's Hong Kong-listed shares fell roughly 0.6% on Monday. Guolian Securities dropped nearly 3%, while Haitong International lost 1.1%.

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

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)