ReutersSir Keir Starmer is due to meet President Xi Jinping at the G20 summit in Brazil, with the government saying it wants “stable and pragmatic engagement” with China.
The meeting will be the first time a UK prime minister has met the Chinese president in person since 2018.
Sir Keir will say it is right to engage with China on “areas of mutual cooperation” such as international stability, climate change and economic growth.
But Downing Street said he would also “be firm on the need to have honest conversations on areas of disagreement”, and that engagement would be “rooted at all times in the UK’s national interests”.
Speaking to reporters on his way to the summit in Rio de Janeiro, Sir Keir said “shoring up support for Ukraine” was top of his agenda.
Downing Street says the prime minister will urge other G20 nations to step up their support for Ukraine or face “unfathomable consequences” if Russia is allowed to be victorious.
The summit follows large-scale missile and drone attacks by Russia across Ukraine over the weekend, and will take place as the conflict approaches its 1,000th day.
And it comes amid reports that the US has authorised the use of long-range missiles it supplies to Ukraine to strike Russia.
The weapons have so far only been used by Kyiv on Russian-occupied targets within its own territory.
The UK has also supplied Ukraine with its Storm Shadow missiles and defence officials and ministers have for months been making the case for Kyiv to be permitted to use them to hit targets inside Russia. But they were not willing to act alone – and had been waiting for the White House to change its mind.
Downing Street has not directly commented on the reports about the US decision.
Downing Street said Sir Keir and President Xi were expected to meet on Monday.
The pair spoke over the phone in August, after Labour’s election victory the previous month, but they have not yet met in person.
China’s military support for Russia’s war in Ukraine has prompted criticism from the UK and other Western countries.
The prime minister said: “It is in the UK’s best interests to engage on the global stage – whether that’s building strong and fruitful partnerships with our closest allies or being frank with those whose values differ from our own.
“Close cooperation with the world’s leading economies is vital to secure investment into the UK, and create the jobs needed to catalyse growth.”
Foreign Secretary David Lammy held talks with his Chinese counterpart Wang Yi, as well as Vice Premier Ding Xuexiang, in Beijing last month.
The last British prime minister to meet President Xi was Theresa May, who hailed a “golden era” for UK-China relations during her 2018 visit to the country.
However, since then there have been tensions over issues including China’s treatment of the Uyghur minority group in Xinjiang and pro-democracy activists in Hong Kong.
Last year, then-Conservative Prime Minister Rishi Sunak said China was “the biggest state-based threat to our economic security”.
But like the current government, he also said it was necessary to engage with China on issues like climate change and the global economy.
The G20 is made up of 19 countries with the largest global economies as well as the African Union and the European Union.
Downing Street said the prime minister would also be focusing on building partnerships that increase economic growth and security during the summit, accelerating the climate transition and use of clean power, and supporting the economic development of developing countries.