MI5 will double its investigations into China in the face of the “game-changing” threat posed by the ruling Communist Party, the head of the security service has said.

Director general Ken McCallum said MI5 is running seven times as many probes into China as it was four years ago, and plans to “grow as much again” to tackle the widespread attempts at inference which pervade “so many aspects of our national life”.

In an unprecedented joint appearance with the director of the FBI, Christopher Wray, at MI5’s Thames House headquarters in London on Wednesday, an audience of business leaders and academic chiefs also heard warnings that if China invaded Taiwan as feared this could “represent one of the most horrific business disruptions the world has ever seen”.

Mr McCallum said they were speaking for the first time in public together to “send the clearest signal we can on a massive shared challenge: China”, adding: “The most game-changing challenge we face comes from the Chinese Communist Party (CCP). It’s covertly applying pressure across the globe … We need to talk about it. We need to act.”

He said MI5 had “already more than doubled our previously-constrained effort against Chinese activity of concern.

“Today we’re running seven times as many investigations as we were in 2018.

“We plan to grow as much again, while also maintaining significant effort against Russian and Iranian covert threats.”

Later he told reporters: “We plan to double again,” adding: “China is the most game-changing threat in the sense that it pervades so many aspects of our national life.”

Describing the CCP’s use of clandestine, coercive or corrupt methods to launch “deceptive” plots to buy and exert influence as well as the use of “sophisticated interference efforts”, Mr McCallum said the threat was a “co-ordinated campaign on a grand scale”, while Mr Wray described it as “breathtaking”.

If China was to try to “forcibly take over Taiwan” it would “represent one of the most horrific business disruptions the world has ever seen”, the FBI boss said, adding that he was “confident in saying that China is drawing all sorts of lessons from what’s happening with Russia and its invasion of Ukraine”.

“I don’t have any reason to think their interest in Taiwan has abated in any fashion,” he said.

The FBI has substantially increased its investigations into China in recent years, with about 2,000 probes at present and a new one opened every 12 hours on average, Mr Wray told reporters.

Describing the threat as a “complex, enduring and pervasive danger” to “innovative businesses” which was “getting worse” and was “even more serious” than many realise, he said: “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 our nations, along with our allies in Europe and elsewhere.”

Mr Wray told the audience Beijing’s administration is “set on stealing your technology, whatever it is that makes your industry tick, and using it to undercut your business and dominate your market”.

Over the last year the UK has shared intelligence about Chinese cyber threats with 37 countries and in May disrupted a “sophisticated threat” against aerospace companies, Mr McCallum said, adding that visa reforms had seen 50 students linked to Chinese military, the People’s Liberation Army, leave the UK.

He cited a series of examples of Chinese interference, including the case of a British aviation expert who was approached online and offered an “attractive employment opportunity” which saw him twice travel to China to be “wined and dined” before being asked, and paid for, technical information on military aircraft by a company which was a front for Chinese intelligence officers.

“That’s where we stepped in”, Mr McCallum said.

He told how engineering firm Smith’s Harlow was forced into administration in 2020 after it entered into a deal with a Chinese company which abandoned the partnership once it shared vital technology.

Mr McCallum also pointed to the security alert issued to Parliament earlier this year over Christine Lee, a suspected Chinese spy accused of targeting MPs, as he said operations which aimed to amplify pro-CCP voices and silence those which question its authority “need to be challenged”.

In the US, Mr Wray said the Chinese government tried to stop a critic and former Tiananmen Square protester from winning a congressional election in New York by manufacturing controversy and even suggesting staging a car accident to injure the candidate.

“China has for far too long counted on being everybody’s second-highest priority,” Mr Wray said, adding: “They are not flying under the radar any more.”

The UK needs to become a “harder target” by becoming more aware of the risks, Mr McCallum said, as he highlighted how proposed laws in the National Security Bill, if passed, will provide a “long-needed and essential shift in powers to combat state threats”.

But he said the aim was “not to cut off from China”, adding: “We want a UK which is both connected and resilient.”

The security chiefs were discussing the activities of certain parts of the Chinese state and “not the Chinese people” who they “wholeheartedly welcome”, Mr McCallum said, adding: “If my remarks today elicit accusations of Sinophobia, from an authoritarian CCP, I trust you’ll see the irony.”