Volodymyr Zelensky is expected to join the G7 summit in Hiroshima in person as Rishi Sunak and allies seek to turn up the pressure on Russia.
The Ukrainian president will attend the summit in Japan on Sunday, it is understood, as the Prime Minister warned Vladimir Putin “we’re not going away”.
It would potentially bring Mr Zelensky into contact with India’s Narendra Modi and Brazil’s Luiz Inacio Lula da Silva, who have not opposed the invasion to the extent that western allies would like.
Earlier, Mr Sunak announced Britain is banning Russian diamonds in a new wave of sanctions to pressure Russia that will also prohibit imports of Russian-origin copper, aluminium and nickel.
There has been uncertainty whether the EU will go as far as Britain on diamonds, with the trade particularly lucrative for Belgium, but Mr Sunak said he is “hopeful and confident” allies will follow.
Having signed a new defence and security pact with Tokyo, Mr Sunak will visit the A-bomb dome ruin with his Japanese counterpart, Fumio Kishida, to reflect on the atomic devastation in the Second World War.
Talks between G7 allies, including US President Joe Biden and France’s Emmanuel Macron, will centre on military and economic support for Ukraine as they seek to show a united front.
Mr Sunak told Sky: “Russia needs to know that we and other countries remain steadfast in our resolve to support Ukraine, not just in the here and now with the resources it needs to protect itself, but for the long term as well.”
He added to ITV News: “They can’t just outlast us in this conflict.
“One of the common topics of conversation I’ll be having and have been having with my fellow leaders is about the longer-term security agreements that we put in place in Ukraine, to deter future Russian aggression.”
The UK is also preparing new individual sanctions against 86 people and companies to apply further pressure on the Russian president and his supporters.
The diamond export industry was worth more than £3 billion to Russia in 2021 but No 10 conceded direct imports have been low since the UK sanctioned state-owned miners Alrosa last year.
Britain’s move will add pressure on any hesitant allies to follow suit.
Mr Sunak told the BBC: “I’m hopeful and confident that our partner countries will follow as they have done when we’ve done this previously, that will make the sanctions more effective, ensure that Russia pays a price for its illegal activity.”
The three-day summit – also including Germany, Italy and Canada – will include talks on economic stability and the security of Taiwan in the face of Chinese aggression.
Mr Sunak’s official spokesman said: “Guarding against economic coercion is something that the Prime Minister is pushing for.”
On Sunday, Mr Sunak will meet his Indian counterpart, Narendra Modi, who is attending the summit as a guest.
Mr Modi, who has been close to the Russian president, has remained neutral on the Russian invasion, calling for peaceful dialogue to end the war.
The Prime Minister told reporters travelling with him in Japan that he has seen “positive” steps from India in its stance and stressed the need to keep up the dialogue.
“One thing we have to keep doing is talking to countries like India and also Brazil, that is going to be in that second part of the summit which is a good thing,” Mr Sunak told reporters.