The meeting at this week's G20 summit in Japan does not represent a normalisation of relations, the spokesman told reporters.
"The prime minister's position on Salisbury and Russia's wider pattern of malign behaviour is well-known.
As she has said, we remain open to a different relationship but that can only happen if Russia desists from activity that undermines international treaties and our collective security," the spokesman said.
"This meeting is an important opportunity to deliver this message leader-to-leader to ensure the UK's position is fully understood ... This meeting does not represent a normalisation of relations."
Britain's relations with Russia fell to a post-Cold War low after the poisoning of a former double agent and his daughter with a nerve agent in Salisbury, southern England last year.
Britain blamed Russia, which denied any involvement.