Image copyright Getty Images Image caption Coastal urban populations in China are among those most at risk, the study says

A British aid charity is warning that by 2060 more than a billion people worldwide will live in cities at risk of catastrophic flooding as a result of climate change.

A study by Christian Aid says the US, China and India are among the countries most threatened.

It says the Indian cities of Kolkata and Mumbai will be most at risk.

The eight most vulnerable cities on the list are all in Asia, followed by Miami in the US.

The report urges governments to take action to reduce global warming and invest in disaster reduction programmes.

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Dr Alison Doig, the report's author, told the BBC World Service that people living in large coastal cities were particularly at risk.

"I think it's cities like Kolkata, Dakar, the big mega-cities of the south and the emerging economies where the people are most vulnerable to exposure to sea-level rises and to higher rain events," she said.

"Flooding in these cities can cause massive damage, but can also threaten life."

Dr Doig warned that Florida was likely to suffer extensive flooding.

Image copyright Getty Images Image caption Miami in Florida suffered flooding in September last year

"The whole of Florida is totally vulnerable," she said.

"It is so low-lying, it is virtually swampland reclaimed. So significant climate change... raising water half to a full metre this century, will take out an awful lot of Florida and a significant amount of Miami."

The study says that the priority should be to rapidly reduce carbon emissions and limit temperature increase by switching from fossil fuels to low-carbon energy.

It says vulnerable communities must be helped by better protecting homes and livelihoods.

The study also calls for international systems to help communities recover from devastating storms and floods.