BEIJING -- The decision by the National People's Congress (NPC) on improving the electoral system of the Hong Kong Special Administrative Region (HKSAR) has a firm constitutional basis, sufficient legal ground, and unchallengeable legal force, a Chinese official said Friday.

Zhang Yong, deputy head of the Commission for Legislative Affairs of the NPC Standing Committee, made the remarks at a press conference.

Shedding light on the constitutional basis of the NPC decision, Zhang said the Constitution of the People's Republic of China is applicable in the HKSAR and has the supreme and universal legal force in the country.

The fundamental aim of the "one country, two systems" policy is to safeguard national unity and territorial integrity, and maintain the prosperity and stability of Hong Kong, Zhang added.

The HKSAR Basic Law, which was formulated in accordance with the Constitution, lays down principles and preconditions for the region's functioning systems, Zhang said, noting that such principles include "the administration of Hong Kong by Hong Kong people" with patriots as the main body and high degree of autonomy that comes from the authorization by the central authorities.

The Constitution and the Basic Law together constitute the constitutional basis of the HKSAR under "one country, two systems," which is the foundation of all systems that are implemented in the HKSAR, Zhang said.

Relevant systems and mechanisms shall be amended and improved if they are not in conformity with the constitutional basis, or breach or even damage the constitutional basis, Zhang added.

The decision of the NPC has sufficient legal ground as the Constitution authorizes the NPC to determine the establishment of a special administrative region and its systems, and the electoral system of the HKSAR is the essence of the region's political system, Zhang said, adding that the central authorities have the power of decision on systems involving HKSAR.

The NPC decision is of unchallengeable legal force, and it should be strictly followed by the NPC Standing Committee when amending Annex I and Annex II to the Basic Law, and by the HKSAR when revising relevant local laws, Zhang said.