BEIJING — Communist China will take “resolute and strong measures” should Speaker Pelosi proceed with reported plans to visit Free China, the Foreign Ministry at Beijing said Tuesday.

Ms. Pelosi, who is second in the line of succession to the presidency, in August is due to visit the self-governing island that China claims as its own territory, according to a report in the Financial Times.

She was originally scheduled to visit in April but had to postpone after she tested positive for Covid.

Ms. Pelosi would be the highest ranking American lawmaker to visit the close American ally since Speaker Gingrich, her predecessor, traveled there 25 years ago.

Red China has vowed to annex Taiwan by force if necessary, and has advertised that threat by flying warplanes near Taiwanese airspace and holding military exercises based on invasion scenarios. 

It says those actions are aimed at deterring advocates of the island’s formal independence and foreign allies — principally the United States — from coming to its aid, more than 70 years after the sides split amid civil war.

A visit by Ms. Pelosi would “severely undermine China’s sovereignty and territorial integrity, gravely impact the foundation of China-U.S. relations and send a seriously wrong signal to Taiwan independence forces,” a Foreign Ministry spokesman, Zhao Lijiang, said at a daily briefing.

“If the U.S. were to insist on going down the wrong path, China will take resolute and strong measures to safeguard its sovereignty and territorial integrity,” Mr. Zhao said.

China in recent days has also ratcheted up its rhetoric over American arms sales to Taiwan, demanding the cancellation of a deal worth approximately $108 million that would boost its armed forces’ chances of survival against its much bigger foe. China has the world’s largest standing military, with an increasingly sophisticated navy and a huge inventory of missiles pointed across the 100 mile-wide Taiwan Strait.

“The Chinese People’s Liberation Army … will resolutely thwart any form of interference by external forces and separatist plots of ‘Taiwan independence,’” the Defense Ministry said in a statement posted on its website Tuesday.

While Washington maintains a policy of “strategic ambiguity” over whether it would defend Taiwan in a conflict with China, American law requires it must ensure the island has the means to defend itself and consider threats to its security as matters of “grave concern.”

Washington maintains only unofficial relations with Taiwan in deference to Beijing, but is the island’s strongest political ally and source of defensive arms.

Mr. Zhao gave no details about what potential actions China might take in response to Ms. Pelosi’s visit, but Beijing has generally used military flights and war games to indicate its discontent. 

Chinese pilots have also been accused of aggressive action toward surveillance aircraft from America and its allies operating in international airspace off the Chinese coast, while using lasers and other methods to harass foreign warships in the South China Sea.

China’s most serious threat against Taiwan came in 1995-96, when it held military exercises and lobbed missiles into waters north and south of the island in response to a visit to America by President Lee Teng-hui.