Christian Aid said that cities including London, Beijing and New Delhi face mounting risks from drought driven by climate change.
Without action, London could run out of water within 25 years and a severe drought could cost the capital’s economy £330 million ($404m) a day, the Christian Aid report warned.
Failure to meet promises on tackling climate change would be an “act of monstrous self-harm”, Cop26 president Alok Sharma said on Monday.
The plea from Mr Sharma came as Christian Aid warned it was the poorest, in places such as Harare in Zimbabwe and Kabul in Afghanistan, who would most feel the harmful effects of climate change.
London receives about half the rain that falls in New York City, and climate change will increase the frequency and intensity of droughts in the region, the report said.