Theresa May will make it “absolutely clear” to Vladimir Putin that the two suspects in the Salisbury nerve agent attack should face justice.
In her first substantial meeting with the Russian president since the March 2018 Novichok attack, Mrs May will tell him he needs to halt a pattern of behaviour that “undermines our collective security”.
The UK believes Moscow’s GRU military intelligence agency was behind the Salisbury attack on former spy Sergei Skripal and his daughter Yulia.
Both survived the poison in Salisbury but in July 2018 Dawn Sturgess died after coming into contact with Novichok which is believed to have been in a perfume bottle.
Scotland Yard and the Crown Prosecution Service believe there is sufficient evidence to charge two Russians – known by the aliases Alexander Petrov and Ruslan Boshirov – with offences including conspiracy to murder over the attack on the Skripals.
Online investigation group Belingcat said Boshirov is actually the highly decorated Colonel Anatoliy Chepiga, and Petrov is a military doctor called Alexander Mishkin.
The issue will loom large over the talks between the Prime Minister and Mr Putin in a meeting on the margins of the G20 summit in Osaka.
It is the first formal bilateral meeting since the Salisbury attack put the UK-Russia relationship into the diplomatic deep freeze, although the two leaders did speak briefly at last year’s G20 in Argentina.
Mrs May told reporters accompanying her to Japan that the meeting gave her the chance to deliver a “very clear message, leader to leader”.
Downing Street has insisted the meeting does not represent a normalisation of the relationship with Moscow.
Mrs May said: “We would be open to a different relationship with Russia, but if that is going to happen then Russia needs to stop its activity that undermines international treaties and undermines our collective security like what happened on the streets of Salisbury.
“What this meeting enables me to do is to give that very clear message, leader to leader.”
Asked by PA whether handing over the Salisbury suspects would be a condition for bringing Russia back in from the cold, Mrs May said: “We have identified two individuals, we believe that they should be brought to justice.
“It’s a longstanding position that Russia does not allow the extradition of its nationals.
“But there are European arrest warrants out for these individuals and if they step foot outside Russia then we will be making every effort to ensure that they are brought to justice.
“I’m going to make absolutely clear the position the UK takes in relation to what happened in Salisbury.
“We believe these individuals should be brought to justice.”
The UK expelled 23 Russian diplomats Mrs May claimed were undeclared intelligence officers following the Salisbury attack and international allies including the US followed suit.
Mr Putin said in an interview with the Financial Times: “Listen, all this fuss about spies and counterspies, it is not worth serious interstate relations. This spy story, as we say, it is not worth five kopecks. Or even five pounds, for that matter.
“And the issues concerning interstate relations, they are measured in billions and the fate of millions of people. How can we compare one with the other?”
He said the security agencies should deal with it, that UK businesses wanted to continue working with Russia, and that he believed the UK and his country were interested in fully restoring relations.
“At least I hope that a few preliminary steps will be made. I think it would be easier for Mrs May, maybe, because she is leaving and is free to do what she thinks is right, important and necessary and not to bother about some domestic political consequences,” he said.
Mr Putin asked if anyone had died, and when told someone had, he said it was a tragedy, but asked: “What do we have to do with it?”
He added: “Treason is the gravest crime possible and traitors must be punished. I am not saying that the Salisbury incident is the way to do it. Not at all. But traitors must be punished.”
At the G20 summit Mrs May will also urge governments and tech firms to do more to prevent terrorists live-streaming atrocities on social media.
She will say: “We should do all we can to bring the best minds together across industry to develop technology to tackle the misuse of live-streaming. We’ve seen the damage when terrorists can advertise into people’s homes – now we mustn’t let them broadcast their atrocities in real time.”
The summit continues on Saturday, when Mrs May will have more potentially controversial meetings, with Saudi Crown Prince Mohammed bin Salman and Turkish President Recep Tayyip Erdogan.