CITIBANK will be the first foreign bank to open a major headquarters in China, marking a new era in the highly-regulated mainland. The People's Bank of China, which previously only allowed foreign banks' branch or representative offices, has approved Citibank's bid to base its China headquarters in Shanghai. The decision sets a precedent which is expected to trigger similar applications by other banks. ''By moving the decision-making centre from Hong Kong to Shanghai . . . we can quicken our response in the fast-changing banking environment,'' said a bank spokesman. The headquarters will help speed up integration with the mainland economy and enable the bank to better assist foreign companies investing in China. The bank has two branches and three representative offices in China and now plans to convert existing offices into branches and build up a branch network in coastal areas. Foreign banks' business in China is restricted to foreign currency dealings.