China has accused British lawmakers of "gross interference" in its internal affairs as a group of MPs visit Taiwan.
Members of the Foreign Affairs Committee arrived on Tuesday and have met high-level officials, including Taiwan's foreign minister, Joseph Wu.
Taiwan is self-ruled – but China sees it as a breakaway province that will eventually unite with it.
A controversial visit in August by senior US politician Nancy Pelosi enraged Beijing.
China responded to that visit by holding their biggest-ever military exercises in the seas around Taiwan, and also blocked some trade with the island.
Mrs Pelosi is the US Speaker of the House of Representatives – second in like to the presidency – and was the most senior US politician to visit Taiwan in 25 years.
She said at the time that China couldn't "prevent world leaders or anyone from travelling to Taiwan".
On Thursday, the Chinese embassy in the UK said the MPs' visit to the "Taiwan region of China" took place despite Beijing's "firm opposition".
"This is a flagrant violation of the one-China principle and a gross interference in China's internal affairs," a spokesperson said.
The statement added that any moves to undermine China's interests would be met with "forceful responses".
The comments follow a speech by UK Prime Minister, Rishi Sunak, earlier this week in which he said that the so-called "golden era" of relations with China was over.
Mr Sunak said the closer economic ties of the previous decade had been "naïve" – but added that China's global significance could not be ignored.
"We recognise China poses a systemic challenge to our values and interests, a challenge that grows more acute as it moves towards even greater authoritarianism," he said.
The MPs' visit is one of a number of recent incidents to cause friction in UK-Chinese relations.
They include the detention of a BBC journalist who was covering protests in China, and the involvement of a senior Chinese diplomat in violence against protesters outside the country's Manchester consulate.
On Wednesday, the Chinese ambassador to the UK was summoned to the Foreign Office over the journalist's arrest.
The Foreign Affairs Committee – which is separate from the government and made up of politicians from different parties – has held discussions with their Taiwanese counterparts over issues including security.
Taiwan's Ministry of Foreign Affairs tweeted that Mr Wu held a banquet for the UK delegation and spoke about "increasing authoritarian threats" as well as "worrying issues at home and abroad".
Their visit is part of an investigation into Britain's shifting economic and political focus towards the Indo-Pacific region, following its exit from the European Union.
The committee met Taiwan's Premier Su Tseng-chang on Thursday and is due to see President Tsai Ing-Wen on Friday.
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