Washington and Beijing have reached a landmark agreement that would allow US regulators access to audits of Chinese companies that are listed on American exchanges, in a deal that would halt the threatened delisting of about 200 stocks that trade on Wall Street.

The announcement by US and Chinese regulators is a breakthrough in a longstanding impasse. Beijing has not allowed foreign regulators to inspect Chinese company audits, citing a desire to protect state secrets. The US has said it will force Chinese companies to leave New York stock exchanges if they do not comply with US audit rules.

The Public Company Accounting Oversight Board, the US auditor watchdog, said on Friday it would have the power to select the companies, audit engagements and potential violations it inspects and investigates, without consulting Chinese authorities.

Under the deal signed by the PCAOB, the China Securities Regulatory Commission and China’s finance ministry, PCAOB inspectors could be on the ground in Hong Kong by mid-September to begin inspections, the agency said.

Despite the agreement, US regulators were cautious about the success of the deal.

“Make no mistake, though: The proof will be in the pudding,” said Gary

Gensler, chair of the US Securities and Exchange Commission, in a statement. “This agreement will be meaningful only if the PCAOB actually can inspect and investigate completely audit firms in China.

‘If it cannot, roughly 200 China-based issuers will face prohibitions on trading of their securities in the US if they continue to use those audit firms,” he added.

The CSRC said the agreement “establishes a co-operation framework in line with the authorities’ respective laws” and “is an important step forward by regulators in China and the US towards resolving the audit oversight issue that concern mutual interests”.

The agreement will involve Big Four audit firms’ working files that are prepared in mainland China being inspected by PCAOB officials in Hong Kong, according to people with knowledge of the details. Several people close to the matter cautioned that the pilot would have to go smoothly in order for the