A stark warning over the monumental cost of flooding in some of the world’s biggest coastal cities including New York (pictured), Mumbai and Guangzhou was issued by the OECD yesterday.

A mind-boggling $1trillion (£0.6trn) worth of losses could result from future flooding if nations do nothing to boost their flood defences in the face of rising sea levels caused by climate change.

What’s more the international group’s research raises the alarm about the future vulnerability of five cities not typically seen as at risk from this issue including Italy’s Naples and Egyptian port Alexandria.

The report called ‘Future Flood Losses in Major Coastal Cities’ projects the cost of flooding to coastal areas in 2050 could be nearly nine times greater than the total estimated losses in 2005, rising from roughly $6 billion (£3.8bn) per year to $52 billion (£33bn) in four and a half decades.

The study which builds on past OECD rankings of global port cities finds that even a moderate rise in sea level would lead to soaring losses if countries do nothing about the problem because flood defences have been designed for past conditions.

Dr Stephane Hallegatte, lead author of the study said: “There is a limit to what can be achieved with hard protection: populations and assets will remain vulnerable to defence failures or to exceptional events that exceed the protection design.”

Out of the 136 cities fingered for concern, three American cities – Miami, New York City and New Orleans – are responsible for 31% of the losses, which the analysts put down to their high wealth and low protection level. If Guangzhou is added to this, the four top cities explain 43% of global losses as of 2005, according to the OECD.