The case study analysed extensive data to estimate the population-wide impact of rainfall and floods on mortality in Mumbai, a city of 22 million people.

It provides a stark warning for other coastal megacities worldwide: climate-driven floods are an "escalating public health threat" as the heating planet is bringing heavier rains and causing sea levels to rise.

South Asia's annual monsoon rains sustain more than a billion people, but climate change is making them increasingly erratic and deadly, with poor infrastructure exacerbating the impact.

"Rainfall is leading to more deaths in Mumbai," says the report, which was published in Nature.

It notes that "vulnerable groups such as the poor, young children and women" are experiencing the greatest impacts.

Rainfall-driven flooding in Mumbai is responsible for roughly eight percent of all deaths during the monsoon season, representing 2,300-2,700 lives lost annually between 2006 and 2015, the researchers found, a toll "comparable to cancer deaths".

Mumbai is surrounded by the sea on three sides, and inadequate drainage means that floods are made worse when the tide is up.

"Intense bursts of rain, and especially intense bursts combined with high tides are the deadliest", says the report.

The impacts reflect divisions in wealth, according to the study, noting that 85 percent of those who "die from rainfall live in slum areas".

"We have witnessed the impact of rainfall and flooding time and again -- traffic accidents, electrocutions, drownings from rising flood waters," said co-author Ashwin Rode.

"But with poor drainage and sanitation systems, standing floods can also trigger many after-effects... diseases like dengue, diarrhoea, malaria can flourish."

The report warns that without action, rising seas will amplify rainfall-driven deaths "by as much as 20 percent over the coming decades".