China and the US were expected to announce ratification jointly before or at the weekend’s G20 Hangzhou summit.
In a landmark deal struck in December, countries agreed to cut emissions enough to keep the global average rise in temperatures below 2C.
The Paris deal is the world’s first comprehensive climate agreement. It will only come into force legally when it is ratified by countries producing 55% of global carbon emissions. When the US follows, it will bump the tally up to 40%.