China is facing an uphill battle to maintain an orderly depreciation of the yuan as investors pile up bearish bets against the currency outside the mainland.

The gap between the yuan’s value against the dollar in the domestic market and in what is known as the offshore market in Hong Kong, has been widening in recent days. On Wednesday, this so-called spread reached 0.0333, its widest since the beginning of October (apart from the day after the U.S. election), although it narrowed a touch on Thursday.

While...