China's brokerages, enjoying their most profitable year in a decade amid the country's stock rally, must contribute part of their annual revenue toward a fund to protect investors, the regulator said today in a statement.

Brokers must pay between 0.5 percent and up to 5 percent of the revenue to the fund, the China Securities Regulatory Commission said on its website. The fund will be collected by August 30 this year, the regulator said.

The China Securities Regulatory Commission wants to protect investors, operating 107.1 million accounts at the end of June, in the stock market. The move may add to the operating costs of brokerages, forcing smaller firms to merge in the world's best-performing stock market this year.

"Brokerages profits will be hammered, especially the weaker ones because paying 5 percent of their revenue is a big cost," said Yi Yangfang, who helps manage about $5 billion at GF Fund Management Co in southern China's Guangzhou city. "The government will try to encourage a wave of mergers and acquisitions in the brokerage industry."

The percentage of revenue paid to the fund will be based on the brokerage's ratings givens by the regulator, according to today's rules. The securities watchdog didn't disclose how it rated the brokerage companies or the rating results.