The Bank of China (BOC) has denied accusations by state television that it helped clients evade limits on money transfers overseas, in a rare attack by the government broadcaster against a state-backed institution.

In a report, China Central Television compared BOC to an "underground" bank, with a secretly filmed official shown telling a supposed client that the BOC could "launder" your money.

BOC is one of the country's "Big Four" state banks, and traditionally the main foreign exchange bank.

In a statement to the Hong Kong stock exchange, where it is listed, the bank said media reports referred to a pilot scheme which allows cross-border transfers for investment immigration and purchases of overseas property.

"The relevant branches of the bank have put in place robust business operation procedures in compliance with the relevant regulatory and anti-money laundering requirements," the statement said late Wednesday.

"Precautionary measures are also adopted to prevent business risks, including reviewing transaction documents on a case-by-case basis and reporting each transaction," it added.

The television report said: "Even dirty money can be processed through the foreign exchange channel for investment immigration purposes, in that case, what's the difference between a bank and an illegal, underground money house?"

China tightly controls the exchange rate of the yuan -- also known as the renminbi (RMB) -- and limits capital flows into and out of the country.

Current regulations limit individuals to converting the equivalent of $50,000 annually.

But the country is gradually moving to implement financial reforms, make its exchange rate more flexible and open its capital account for investment and financial transactions, rather than trade-related ones.

BOC argued that many commercial banks offer the same type of transfer service.

The bank's stock closed down 0.86 percent in Hong Kong and fell 0.78 percent in Shanghai, where it is also listed.