China has warned that Taiwan's President Chen Shui-bian "risks war" if he launches any independence bid.
The comments comes as Taiwan is preparing for elections next year
Separatist forces would pay a "high cost" if they pushed ahead with formal independence, Chinese media quoted a senior official as saying.
Beijing has long threatened action against Taiwan, which it sees as a renegade province.
Correspondents say the threat, made four months before Taiwan elections, is rarely put in such aggressive terms.
The remarks from Wang Zaixi, Vice Minister of China's Taiwan Affairs Office, are the most hardline public comments since Taiwan last held presidential elections in 2000.
President Chen, who has formally declared his intention to seek re-election in March's presidential elections, has signalled that he wants to make the debate over whether or not Taiwan should declare independence the main issue of the election campaign, a move sure to further anger Chinese authorities.
BBC correspondent Francis Markus says one of China's main concerns is that President Chen appears to be gaining popularity with the voters.
He is already a hate figure in Beijing because of his party's support for formal independence and now he seems to be hardening his stance on a new constitution for the island and a referendum on Taiwan's future, our correspondent adds.
China has threatened to reclaim Taiwan through military means if the island ever formally declares independence.
It regards Taiwan as a part of China although it has been governed separately since 1949.
Taiwan is formally recognised by fewer than 30 countries and was replaced at the United Nations by China in 1971.