Chinese President Xi Jinping has urged the people of Taiwan to accept it "must and will be" reunited with China.
In a speech marking 40 years since the start of improving ties, he reiterated Beijing’s call for peaceful unification on a one-country-two-systems basis.
However, he also warned that China reserved the right to use force.
While Taiwan is self-governed and de facto independent, it has never formally declared independence from the mainland.
Beijing considers the island to be a breakaway province and Mr Xi’s comments are in line with China’s long-standing policy towards reunification.
Under the "one country, two systems" formula, Taiwan would have the right to run its own affairs; a similar arrangement is used in Hong Kong.
In his speech, Mr Xi said both sides were part of the same Chinese family and that Taiwanese independence was "an adverse current from history and a dead end".
Taiwanese people "must understand that independence will only bring hardship," Mr Xi said, adding Beijing would never tolerate any form of activity promoting Taiwanese independence.
Instead, unification was "an inevitable requirement for the great rejuvenation of the Chinese people", he argued.
He also stressed that relations with Taiwan were "part of China’s domestic politics" and that "foreign interference is intolerable".
Beijing "reserves the option of taking all necessary measures" against outside forces that interfere with peaceful reunification and Taiwanese separatist activities.
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