Theresa May has upbraided Vladimir Putin for the Salisbury poisoning, calling it a “truly despicable act”, during a frosty one-to-one meeting at the G20 summit in Osaka that is likely to be their last encounter.

After exchanging a handshake, during which May appeared stern, the pair held the first half of their 80-minute meeting alone, with only translators in the room.

A senior government official confirmed that before the meeting, the prime minister had read a defiant interview with the Russian president in the Financial Times, in which he declared liberalism to be “obsolete”.

According to a Downing Street spokesperson, she told Putin: “The use of a deadly nerve agent on the streets of Salisbury formed part of a wider pattern of unacceptable behaviour and was a truly despicable act that led to the death of a British citizen, Dawn Sturgess.”

And she confronted Putin with the fact that the UK had gathered “irrefutable evidence that Russia was behind the attack – based on painstaking investigations and cooperation with our allies”.

“She said that this behaviour could never be repeated and that the UK wants to see the two individuals responsible brought to justice,” the spokesperson added.

“The prime minister underlined that we remain open to a different relationship, but for that to happen the Russian government must choose a different path. The prime minister said the UK would continue to unequivocally defend liberal democracy and protect the human rights and equality of all groups, including LGBT people.”

The prime minister had insisted before the pair met that Britain’s relationship with Russia could not return to “business as usual” while the suspects from the Salisbury poisoning remained at large and Moscow continued to carry out disinformation and cyber-attacks.

“It’s not business as usual and it can’t be business as usual with Russia until they stop the sort of acts we have seen them doing around the world,” she told ITV. “We in the UK have seen the use of a nerve agent, a chemical weapon, on the streets of Salisbury.

“But we see other Russian activity around the world – we see the use of disinformation, cyber-attacks – and we need to see Russia desisting from that activity.”

The two leaders met after Putin’s FT interview in which he attacked liberalism and made light of the poisoning of the Russian former spy Sergei Skripal in Salisbury last year.

He said: “Listen, all this fuss about spies and counterspies, it is not worth serious interstate relations. This [Skripal] spy story, as we say, it is not worth five kopeks. Or even £5, for that matter.”

Elsewhere in the interview, Putin said “the so-called liberal idea” had “outlived its purpose”, and criticised open borders and multiculturalism.

He said Russia was taking action against unfettered migration, “whereas the liberal idea presupposes that nothing needs to be done. The migrants can kill, plunder and rape with impunity because their rights as migrants must be protected. What rights are these? Every crime must have its punishment.”

May also told Putin the UK would “continue to unequivocally defend liberal democracy and protect the human rights and equality of all groups, including LGBT people”, in what appeared to be a response to Putin’s uncompromising remarks in the FT interview.

Reporters were admitted to the early moments of the meeting between the pair, before leaving the room to allow them to speak “leader to leader”, as the prime minister called it. She looked serious throughout.

The widow of Alexander Litvinenko, who was poisoned in London in 2006 with a radioactive cup of tea, said she was disappointed May had agreed to meet Putin.

Marina Litvinenko said the prime minster had failed to mention Andrei Lugovoi and Dmitry Kovtun – the two Russian assassins who had poisoned her husband. Putin has refused to extradite the pair. A UK public inquiry in 2016 found them responsible for Litvinenko’s murder. It said Russia’s president had “probably” approved the assassination.

“Mrs May has said nothing about what happened to my husband. He was a British citizen when he died. The two suspects for his murder should be brought to justice.”

Earlier, Putin appeared at a press briefing alongside Donald Trump. When asked by a reporter whether he would challenge Putin over interfering in elections, the US president turned to his counterpart, wagged his finger and said: “Don’t meddle in the election, president.” The pair laughed.

Play Video 1:11 Trump tells Putin: don't meddle in the US presidential election - video

May is also meeting the Turkish president, Recep Tayyip Erdoğan, and the Saudi crown prince, Mohammed bin Salman, at the two-day summit in Japan.

It is one of May’s final outings on the world stage, before she hands over the keys to 10 Downing Street to either Boris Johnson or Jeremy Hunt next month.