China's top securities regulator moved on Monday to tighten its supervision of listed companies' acquisitions, with moves to make disclosures of transactions more transparent and professional advisers more accountable.

The China Securities Regulatory Commission ordered that major shareholders, defined as those who hold more than 50 percent of a listed company's issued stocks, must disclose every 1 percent increase in their stakes during an acquisition process.

If the increase exceeds 2 percent, these shareholders must halt purchases for the rest of the day and the next as well, the commission said.

The previous disclosure rule covered changes of 5 percent or more.

The new rule could reduce price volatility and enhance the fairness of trade, said a CSRC official who declined to be identified.

The commission also said that shareholders with stakes of larger than 30 percent may only increase their holdings by an amount equivalent to less than 2 percent of the total volume in the previous 12 months.

These changes will take effect on March 15.

The changes signal tighter supervision of acquisitions involving listed companies, said Yang Hai, chief analyst at the China Financial Online, a professional stock market information and services company.

"Making a fortune overnight could become very hard for acquiring companies," Yang said. "It could lead to more rational investments."

The CSRC also announced rules covering expert advisory committees for listed company acquisitions.

Committee members have a duty to provide professional advice about legislation, accounting and asset appraisal for listed companies, the commission said.

They will also have the right to vote on whether a company can obtain share-issue approvals before the acquisition verification department makes a final decision.

These experts, who can serve for up to three consecutive years at a time, are nominated by the CSRC. The list of names will be disclosed later. The maximum number of committee members is 35.

Last week, the CSRC Chairman Guo Shuqing called for the acceleration of market-oriented acquisitions of public companies through improved supervision.

"High-quality listed companies with advanced operating systems and a high standard of information transparency can get priority support during the verification procedure for acquisitions," said Guo.

Stricter supervision might dissuade some companies from pursuing acquisitions, which would probably reduce the number of such deals this year, Yang said.