"Any declaration by Taiwan to separate itself legally and permanently from China will be viewed by [the mainland] as a violation of Chinese territorial integrity, hence incurring war," Chu wrote.

He is the candidate for the KMT, which has forged economic agreements with the mainland, easing tensions across the Taiwan Strait. That culminated in a historic meeting last week between present Taiwanese leader Ma Ying-jeou and President Xi Jinping .

But Ma's rapprochement with Beijing met a mixed reception in Taiwan, and Chu is badly trailing in the polls to his DPP counterpart Tsai Ing-wen, whose party has traditionally advocated independence.

The DPP has moderated its stance, and when Tsai visited Washington in June, she said she wanted to maintain the status quo in Taipei-Beijing relations. Still, she refused to endorse the principle of there being only one China. Beijing has claimed sovereignty over Taiwan since the Chinese civil war of the 1940s.

Chu said the DPP was offering the policy goal of maintaining the "status quo"but "shirking the responsibility of providing a workable formula" to achieve it, according to a copy of his prepared remarks for a private appearance at a Washington think tank on Friday.

The DPP quickly accused Chu of diplomatic impropriety and distorting its policy position.

"Mr Chu would be well advised to follow professional diplomatic policy, standard for any serious politician, to not bring domestic differences over foreign policy beyond one's country's shores. Perhaps Mr Chu is not ready for prime time yet," said Michael Fonte, director of the DPP mission in Washington.

Visits to Washington by Taiwanese officials and politicians are diplomatically sensitive. Daniel Russel, the top US diplomat for East Asia, who met Chu, said both presidential candidates had been granted the same level of access.

"We don't play favourites," Russel said. "It's for the voters on Taiwan to decide who their favourite is."