Rishi Sunak is promising to “call out Putin’s regime” at a gathering of world leaders in Indonesia.
The prime minister is travelling to Bali on Sunday afternoon for a summit of the G20 – the world’s biggest economies.
British officials had prepared for this get-together assuming Russia’s president would be there.
The prime minister was expected to join other world leaders in condemning Vladimir Putin to his face.
But Moscow said last week he wouldn’t be attending and the Kremlin would be sending Russia’s foreign minister, Sergei Lavrov, instead.