SHANGHAI: Text message solicitations began arriving on the mobile phones of many of China's wealthy last month, promising access to lucrative wealth management products with yields far above the government's benchmark savings rate. 

One message read: "China Merchants Bank will issue a high interest financing product starting from June 28th to 30th. The product will be 90 days with a 5.5% interest rate. Please call us now." 

A day later came another. "Warm reminder: The interest rate of yesterday's product has been raised to 6%. (Product duration is 90 days). There is limited access to this product. First come first served." 

The offers are not coming from fly-by-night operators but some of China's biggest banks. They are raising huge pools of cash to finance a relatively new and highly profitable sideline business: lending outside the scrutiny of bank regulators. 

The complex way they go about making off-the-balance-sheet loans is at the heart of China's $6 trillion shadow banking industry, which the government is now trying to tame. Efforts to rein in the dodgy lending practices put stock markets worldwide in a tizzy in late June. 

China's regulators - and a fair number of economists, policymakers and investors - worry that legitimate banks are using lightly regulated wealth management products to repackage old loans and prop up risky companies and projects that might not otherwise be able to borrow money. 

Analysts warn that shadow banking is helping fuel the rapid growth of credit in a weakening economy, which could lead to - in a worst-case situation - a series of bank failures. 

"This is the biggest uncertainty I've seen in my 18 years following the China market," Dong Tao, an economist at Credit Suisse, said of shadow banking. "You don't know how banks are deploying capital. And you don't know the credit risks." 

What banks are doing, analysts say, is pressing customers to shift money from the old, regulated part of their operations - savings deposits - into the new, less regulated part consisting of high-yielding wealth management products that can circumvent government interest rate controls and be used to finance high-interest loans to desperate customers. 

China's leaders are so worried about credit risk that last month the country's central bank tightened credit in the interbank market, where banks typically go to borrow money from other banks.