A view of Hong Kong in July. Photo: VCG

Rents for shop space in Hong Kong plunged by 10.5 percent in the third quarter, as months of riots in the city took a toll, local media outlets reported, citing a report by US commercial real estate services CBRE Group Inc.

The decline was the sharpest in 21 years in Hong Kong, one of the world's most expensive cities, according to a report by local news site hkej.com on Tuesday.

The CBRE report further forecast another 5-10 percent fall for shop space rents before the year's end. 

The fall was due to growing cautiousness by retailers over continued riots in the city, as the China-US trade war and the riots cast a negative impact on locals' willingness to spend.

Rents in shopping malls may also take a hit as many of the riots took place near these venues.

In August, retail sales in Hong Kong fell about 23 percent year-on-year to HK$29.4 billion ($3.75 billion), according to the city's Census and Statistics Department.

Hong Kong Financial Secretary Paul Chan Mo-po urged real estate owners and property developers to take various measures, such as cutting rents and offering rent-free periods, to help retailers get through the challenges brought by a slowing global economy and local social incidents.