HONG KONG — A proposal to lease part of a new Hong Kong rail terminal to mainland China and to allow Chinese officers to enforce mainland law there has raised concerns the plan would undermine this city’s legal autonomy.

Hong Kong officials have said that the terminal, which is scheduled to open late next year in the city’s West Kowloon district, needs mainland customs and immigration officers in addition to Hong Kong officers in order to maximize efficiency for travelers. The 85-mile express rail line will connect Hong Kong with the cities of Shenzhen and Guangzhou in the neighboring mainland province of Guangdong.

When Hong Kong, a former British colony, returned to Chinese control 20 years ago, it was promised a high degree of autonomy, including the ability to keep its own legal, political and economic systems. Hong Kong maintains its own borders, and travelers between Hong Kong and mainland China pass through immigration controls similar to those between nations.

Under the Hong Kong proposal announced on Tuesday, customs, immigration and quarantine officers from Guangdong Province would have enforcement powers in about one-quarter of the station’s five-story terminal.