LONDON — The head of the FBI and the leader of Britain’s domestic intelligence agency raised alarms Wednesday about the Chinese government, warning business leaders that Beijing is determined to steal their technology for competitive gain.

The director of the FBI, Christopher Wray, reaffirmed concerns in denouncing economic espionage and hacking operations by China as well as the Chinese government’s efforts to stifle dissent abroad. His speech was notable because it took place at MI5’s London headquarters and alongside the agency’s director general, Kenneth McCallum, in an intended show of Western solidarity.

It also showed the extent to which Mr. Wray and the FBI regard the Chinese government as not only a law enforcement and intelligence challenge, but are also attuned to the implications of Beijing’s foreign policy actions.

“We consistently see that it’s the Chinese government that poses the biggest long-term threat to our economic and national security, and by ‘our,’ I mean both of our nations, along with our allies in Europe and elsewhere,” Mr. Wray said.

Officials described it as the first time that leaders of the FBI and MI5 had shared a stage for a joint speech.

Mr. McCallum said the Chinese government and its “covert pressure across the globe” amounts to “the most game-changing challenge we face.”

“This might feel abstract. But it’s real and it’s pressing,” Mr. McCallum said. “We need to talk about it. We need to act.”

A spokesman for the Chinese embassy at Washington, Liu Pengyu, said in an emailed statement that China “firmly opposes and combats all forms of cyber attacks” and called American  accusations groundless.

In a nod to current tensions between China and Taiwan, Mr. Wray said any forcible takeover of Taipei by Beijing would “would represent one of the most horrific business disruptions the world has ever seen.”

Last week, the American government’s director of national intelligence, Avril Haines, said there were no indications the Chinese president, Xi Jinping, was poised to take Taiwan by military force. Ms. Haines did say Mr. Xi appeared to be planning for that potential action as part of a broader goal of reunification of Taiwan.

After the appearance with his British counterpart, Mr. Wray said that he would leave to others the question of whether an invasion of Taiwan was more or less likely after Russia’s invasion of Ukraine. “I don’t have any reason to think their interest in Taiwan has abated in any fashion,” Mr. Wray said, adding that he hoped China had learned what happens “when you overplay your hand,” as he said the Russians have done in Ukraine.

The FBI director said there are signs the Chinese, perhaps drawing lessons from Russia’s experience since the war, have looked for ways to “insulate their economy” against potential sanctions.

“In our world, we call that behavior a clue,” Mr. Wray said, and  throughout his speech urged caution from Western companies looking to do business in or with China. He said Western investments in China could collapse in the event of an invasion of Taiwan.

“Just as in Russia, Western investments built over years could become hostages, capital stranded [and] supply chains and relationships disrupted,” he said.

President Biden said in May that America would respond militarily if China invaded Taiwan, offering one of the most forceful White House statements in support of Taiwan’s self-governing in decades. The White House later tried to soften the impact of the statement, saying Mr. Biden was not outlining a change in American policy toward Taiwan, a self-governing island that China views as a breakaway province that should be reunified with the mainland.

The embassy spokesman said the Taiwan issue was “purely China’s internal affair” and that when it comes to questions of China’s territory and sovereignty, the country has no room for compromise or concession.