The top Chinese diplomat in the United Kingdom strongly condemns US House Speaker Nancy Pelosi's visit to China's Taiwan region, as the move has crossed the red line of the one-China principle, adding "those who play with fire will get burnt".

Speaking at a press conference in London on Tuesday, Zheng Zeguang, the Chinese Ambassador to the UK, said US Speaker Pelosi's visit to Taiwan "seriously violated the one-China principle and the provisions of the three China-US joint communiqués."

"It seriously infringes upon China's sovereignty and territorial integrity, and seriously undermines the peace and stability in the Taiwan Straits. We firmly oppose such a highly irresponsible, provocative and dangerous act," Zheng said.

Pelosi, a top US Democrat and also the second-in-line for succession of the US President, is visiting Taiwan as part of a tour of Asia, despite repeated warnings from China.

Zheng said: "In 1979, the US government made a clear commitment in the China-US Joint Communiqué on the Establishment of Diplomatic Relations. The United States recognizes the Government of the People's Republic of China as the sole legal Government of China, and the US will only maintain cultural, commercial and other unofficial relations with the people of Taiwan."

Since Speaker Pelosi is the incumbent leader of the US Congress, Zheng said, her visit to Taiwan, in whatever form and under whatever pretext, is "a serious violation of one-China policy as committed by the US government and a major political provocation to upgrade the official contact between the US and Taiwan. It is absolutely unacceptable."

The envoy said raising tension across the Taiwan Straits is "a highly dangerous trick, playing with fire."

Zheng warned the UK "not to dance to the tune of the US and follow in the wrong footsteps of the US", adding both China and the UK can see the benefits of having a productive and constructive relationship.

In response to UK leader hopefuls' recent tough stance on China, the Ambassador urges those UK politicians to stop making up stories and stop spreading fallacies about the so-called China threat and be realistic about the fundamentals of China-UK relationship.

Upon arrival, Pelosi released a statement, saying "Our visit is part of our broader trip to the Indo-Pacific — including Singapore, Malaysia, South Korea and Japan — focused on mutual security, economic partnership and democratic governance."

"Our discussions with Taiwan leadership will focus on reaffirming our support for our partner and on promoting our shared interests, including advancing a free and open Indo-Pacific region. America's solidarity with the 23 million people of Taiwan is more important today than ever, as the world faces a choice between autocracy and democracy."

"Our visit is one of several Congressional delegations to Taiwan – and it in no way contradicts longstanding United States policy, guided by the Taiwan Relations Act of 1979, US-China Joint Communiques and the Six Assurances. The United States continues to oppose unilateral efforts to change the status quo."

The Biden White House in recent weeks has tried to convince Beijing and the world that Pelosi's visit to Taiwan bears no relevance to US policy towards China.

On his July 28 phone call with Chinese President Xi Jinping, US President Joe Biden reiterated that the US's one-China policy has not changed and will not change, and that the US does not support Taiwan independence.

Andrew Mertha, the director of the China Global Research Center at the Johns Hopkins School of Advanced International Studies, told CNBC, "Saying that this is a whole lot of nothing or that the Chinese shouldn't read into it ... Well, anybody who has spent half a minute looking at China knows that they attach some sort of intentionality to everything we do."

He went on to say any suggestion that a visit by someone as important as Pelosi would be seen by Beijing as anything but an in-person expression of American support for Taiwanese independence is unimaginable.

"I think what you really see from China's side, and it's not unreasonable, is that we're kind of pushing the envelope of the One China policy. They're alarmed," Mertha said of Beijing, "and I don't blame them."