Nigel Farage is not a Conservative and should not be part of any Tory frontbench, the former prime minister Theresa May has said, in a speech warning against rising populism. May, who will stand down at the next election, also issued a veiled rebuke to her successors about what she termed “a willingness to row back sometimes on certain aspects of international law”. She used a speech at the Parliamentary Press Galley lunch to also gently mock her successors, saying Liz Truss’s book should be in the fantasy section of the bookshop and Boris Johnson should put his under “current affairs”. But she said she had serious concerns about the rise of authoritarianism and anti-democratic populist leaders across the globe. “Populism seeks to divide. It seeks to divide our societies and it seeks to provide easy answers,” the former prime minister said. The former cabinet minister Jacob Rees-Mogg said earlier this week that he would like to see a Conservative front bench with roles for Farage and the Reform UK leader Richard Tice. But May said that was not the answer. “I absolutely disagree on that. I don’t think Farage is Conservative, I don’t think Tice is a Conservative.” She said she did not believe that the Tories were necessarily on course to lose the next election. “Keir Starmer is not Tony Blair,” she said. “I think I was probably 20 points ahead in most of the election campaign for the 2017 election, and look what happened to that. So against the background of the economy improving, it is not a foregone conclusion.” May also said she believed Britain should remain in the European convention on human rights, including as a bulwark against politicians breaching international law. “Sadly we have shown as a country a willingness to row back sometimes on certain aspects of international law. I think we should stay in something that we helped to create in the first place.” May said she would continue to work on tackling modern slavery after the end of her 27-year parliamentary career and she voiced concerns that Rishi Sunak’s Rwanda scheme would hamper efforts to help and identify victims of human trafficking. “We need a bigger-picture sense in terms of looking at these issues of migration, because it is caught up with so many other issues. Climate change is going to potentially increase the displacement of people. We have to look at all of these issues holistically. The international development role is important and I also think there’s much more the UK can offer. “But remember, the majority of people who are here illegally in the UK … came here legally and overstayed their visas.”