UPDATE 1-China copper imports surge a third in September

* Imports up 31.4 pct at 460,000 tonnes vs 350,000 tonnes in Aug

* Price ratios between LME and Shanghai favour imports

HONG KONG, Oct 13 (Reuters) - China’s monthly copper imports surged a third in September, hitting a 20-month high after staying flat in the previous three months, as price differentials favoured spot purchases and some shipments arrived ahead of the week-long Oct. 1 holiday.

Imports of anode, refined copper, copper alloys and semi-finished copper products stood at 460,000 tonnes in September, the highest since January 2014 and compared to 350,000 tonnes in August, July and June 2015, data from the General Administration of Customs showed on Tuesday.

Arrivals increased 17.9 percent from a year ago. In the first three quarters, imports dropped 5.5 percent from a year ago to 3.39 million tonnes.

“High arrivals in September were linked to good arbitrage ratios. Some shipments also arrived earlier ahead of the week-long Oct. 1 holiday,” said Yao Yao, analyst at Maike Futures Brokerage.

But Yao said the growing imports did not reflect a strong boost in domestic consumption. She expected monthly imports in the fourth quarter to be slow due to the weaker economic growth.

Spot copper prices in China CU-1-CCNMM reached a 2-month high of about 41,200 yuan in September, supported by some production cuts by smelters and expectations that a weaker yuan would increase import costs.

Stronger demand for spot refined copper imports had pushed up premiums for bonded stocks to a one-year high of about $130 a tonne, traders said. Some importers had also bought spot shipments from Asia and Chile.

Bonded stocks in Shanghai, imported metal that has not been assessed for China’s 17-percent value-added tax, were estimated by traders at about 400,000 tonnes currently, compared to about 570,000 tonnes in early August.

Imports of raw material copper concentrate rose 5.2 percent to 1.21 million tonnes in September, compared to 1.15 million tonnes in the previous month, the data showed.

Concentrate imports in September was down 6.2 percent from a year ago. In the first three quarters, imports rose 9.3 percent on-year to 9.33 million tonnes.

Reporting by Polly Yam; Editing by Joseph Radford