Sewage being discharged into West Lake in Hangzhou, Zhejiang province, a site on the UNESCO world heritage list, may be caused by the network being overloaded, according to the authority in charge of the city's sewage system.

China News Service reported on Thursday that untreated sewage has been discharged into part of the lake through a pipe.

The discharge may have been caused by a spill of underground sewage, said Wang Qing, managerial assistant in the production management department of the Hangzhou Water Holding Group.

Wang said most of the sewage discharged around scenic areas comes from hotels and guesthouses, with development of the tourism industry outpacing that of beauty spots.

However, the West Lake scenic area management committee and the water company denied responsibility for maintenance of the sewage pipe network.

Referring to previous experiences, Wang said low-lying areas at scenic spots usually witnessed a sewage overflow during the rainy season.

The city is reconstructing a pumping station at West Lake.