Chinese shares plunged on the first trading session of the new year, with the Shenzhen market suffering its worst day in nine years and the blue-chip CSI 300 index slumping 7 per cent on weak manufacturing data, triggering newly minted circuit breakers. The meltdown follows last year’s market rout that at one stage wiped 45 per cent off the value of the Shanghai stock market, prompting Beijing to introduce several measures to prop up prices — including state-orchestrated buying of shares. Monday’s slide came after an official factory survey showed China’s manufacturing sector shrank for a fifth consecutive month in December. Investors were also nervous about the imminent expiration of a ban on stock sales by large shareholders, imposed at the height of last year’s market meltdown. The circuit-breaker mechanism, which was announced late last year and took effect on Monday, triggers a 15-minute trading suspension for all shares and index futures in Shanghai and Shenzhen if the CSI 300 index falls by at least 5 per cent. A 7 per cent fall halts trading for the rest of the day. The CSI 300 index, which tracks the largest companies traded in Shanghai and Shenzhen, hit the 5 per cent limit at 1:14pm Shanghai time (05:14 GMT) on Monday and fell an additional 2 per cent within five minutes after trading resumed. The Shanghai Composite Index lost 6.9 per cent, while the small cap-heavy Shenzhen Composite lost 8.2 per cent.