China has lodged a diplomatic protest with India over a visit by a parliamentary delegation from self-ruled Taiwan, the Chinese Foreign Ministry said on Wednesday.
Three Taiwanese parliamentarians, led by ruling Democratic Progressive Party (DPP) legislator Kuan Bi-ling, began their visit to New Delhi on Monday.
Chinese Foreign Ministry spokesman Geng Shuang told a regular press briefing in Beijing that China had lodged "solemn representations" with the Indian government over the issue.
"We have always been resolutely opposed to any country with diplomatic relations with China having any form of official exchanges, or establishment of any official institutions, with Taiwan. This position is consistent and clear cut."
India's ministry of external affairs said the visit was informal.
"Such informal groups have visited India in the past as well for business, religious and tourist purposes. I understand that they do so to China as well. There is nothing new or unusual about such visits and political meanings should not be read into them," the ministry said in a statement.
A DPP official in Taipei said the legislators were scheduled to return to Taiwan on Thursday and were also in India to visit Taiwanese companies such as China Steel, Taiwan's biggest steel maker which has a plant there.
Despite efforts by China and India to improve ties over recent years, deep suspicions remain, especially over a festering border dispute.
China considers Taiwan a wayward province, with no right to formal diplomatic ties with other countries.
Beijing has also increasingly squeezed Taiwan's international space following the election of Tsai Ing-wen as president last year.
China suspects she wants to push for the island's formal independence. Tsai says she wants to maintain peace with China.
A longstanding African ally of Taiwan's, San Tome and Principe, switched diplomatic recognition to Beijing in December.
"We have been requiring countries which have diplomatic relations with China to fulfill their commitment to the 'one China' principle," Geng said.
Defeated Nationalist forces fled to Taiwan in 1949 after losing a civil war to the Communists. China has never renounced the use of force to bring Taiwan under its control.