Lai Ching-te, the frontrunner for Taiwan's presidency, has picked Taipei's envoy to the United States - a fluent English speaker with deep connections in Washington - to be his running mate for January's election, sources with direct knowledge said.

Lai, vice president and the ruling Democratic Progressive Party's (DPP) presidential candidate, has almost consistently led opinion polls ahead of an election taking place amid increased Chinese pressure on Taiwan to accept Beijing's sovereignty claims.

Hsiao Bi-khim, 52, who has been Taiwan's de facto ambassador to the United States since 2020, has been considered by party officials, diplomats and Taiwanese media for months to the most likely running mate for Lai.

Now, that decision has been made, with a formal announcement coming on Monday once Hsiao returns to Taiwan from the APEC summit in San Francisco, four sources briefed on the matter told Reuters.

Hsiao, well known in the U.S. capital, can act as a key go-between for Taipei and Washington, the island's most important arms seller and international backer despite the absence of formal ties, one of the sources said.

"It's a crucial role and she has the needed international experience," said the source, speaking like the others on condition of anonymity as they were not authorised to speak to the media.

Vincent Chao, spokesperson for the Lai campaign, declined to comment on Hsiao's role but said an announcement on a running mate would be made on Monday.

"This candidate will reinforce our commitment to the people of Taiwan and the international community that democracy, peace, and prosperity will continue to be our guiding values," he said.

Ivan Kanapathy, who regularly met with Hsiao when he was Deputy Senior Director for Asia at the U.S. National Security Council, told Reuters that Hsiao is a "tireless advocate" for Taiwan. "If Ambassador Hsiao becomes Vice President, her knowledge, experience, and relationships would be invaluable for national security and foreign policy in the next Taiwanese administration," said Kanapathy, now an adjunct professor at Georgetown University.

Hsiao has been "very engaged on Taiwan's defence reform efforts, which are drawing on lessons from Ukraine," he said, pointing to examples such as initiatives for Taiwan to acquire more munitions, including Patriot missiles and drones.

Randall Schriver, the former U.S. assistant secretary of defense for Indo-Pacific Security Affairs, described Hsiao as a "good partner in promoting U.S.-Taiwan relations."

"If Ambassador Hsiao becomes Vice President, there is no doubt this will be to the benefit of U.S.-Taiwan relations," said Schriver, leads the Project 2049 Institute, a think tank. Schriver added that he takes no position in the Taiwan campaign.

The DPP champions Taiwan's separate identity from China. The DPP-led government says only Taiwan's people can decide their future, and has repeatedly offered talks with Beijing but been rebuffed.

Lai has during the past week talked up Hsiao in interviews with local media, without directly confirming he had picked her, calling her "very principled" and "thorough" and that he had always admired her.

Hsiao was born in Japan to a Taiwanese father and American mother and initially worked in the office of then-president Chen Shui-bian, also from the DPP, and then as a DPP lawmaker.

Like Lai, Hsiao is detested by China, which has on two occasions placed sanctions on her, most recently in April, saying she is an "independence diehard".

China's Taiwan Affairs Office on Wednesday referred to Lai and Hsiao as an "independence double act" and that Taiwan's people are "very clear" about what them teaming up means for the "situation in the Taiwan Strait". It did not elaborate. (Reporting by Ben Blanchard and Yimou Lee; Additional reporting by Michael Martina in Washington. Editing by Gerry Doyle)