Keir Starmer challenged Chinese Premier Xi Jinping on Beijing's human rights meeting in a historic meeting, officials said.
The Prime Minister became the first British leader to meet his counterpart since 2018. He raised the case of British national Jimmy Lai, a Hong Kong politician, whose detention since 2020 has sparked worldwide anger.
Mr Starmer also raised Chinese sanctions against UK politicians, Downing Street said - with Tories Iain Duncan Smith and Tom Tugendhat among those previously singled out by Beijing.
Journalists were ushered out of the room as the PM raised the uncomfortable topics. The leaders met in Rio de Janeiro, Brazil, where world leaders have descended for the G20 summit. The meetings bring together the leaders of the world's largest economies amid growing uncertainty on the international stage following the election of Donald Trump.
In an attempted thawing of relations, which has soured in recent years, Mr Starmer proposed a meeting between Chancellor Rachel Reeves and Chinese vice premier He Lifeng.
He also proposed a full bilateral meeting with Chinese premier Li Qiang, either in London or Beijing. Mr Starmer said he is keen to push better economic relations between the two nations.
He said: “I'm keen that my Chancellor should meet with Vice Premier He for the upcoming economic financial dialogue early next year to explore more investment projects and a more level playing field to help our businesses.”
“I'm very pleased that my Foreign Secretary and Foreign Minister Wang met recently to discuss respective concerns including on human rights and parliamentary sanctions, Taiwan, the South China Sea and our and our shared interest in Hong Kong. We are concerned by reports of Jimmy Lai’s deterioration.”
Speaking at the start of their meeting, he said: “We want our relations to be consistent, durable, respectful, and as we have agreed, avoid surprises where possible. A strong UK China relationship is important for both of our countries and for the broader international community.
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“The UK will be a predictable, consistent, sovereign actor committed to the rule of law.” Speaking ahead of the meeting Amnesty International said the "appalling state of human rights across China must be top of the agenda".
A No10 statement said: “The Prime Minister set out that our approach would be consistent, respectful and pragmatic in order to advance these shared goals. On climate in particular, both said that this should be high on the agenda and there was more work to be done to accelerate global progress towards net zero. Both the UK and China have an important role to play in support of the global clean power transition.
“The Prime Minister said that he also wanted to engage honestly and frankly on those areas where we have different perspectives, including on Hong Kong, human rights and Russia’s war in Ukraine. He reiterated that his approach would always be rooted in the national interests of the UK, but that we would be a predictable and pragmatic partner.