The ice sheets in Greenland and Antarctica are melting faster than expected, leading scientists to warn that sea levels around the world could rise by as much as two metres by the end of the century in a worst-case scenario.

This is potentially twice the highest limit suggested by the United Nations' climate science panel’s last major report, according to New Scientist.

The IPCC report in 2013 considered what was "likely" to happen, looking at 17-83% of the range of possibilities.

This new study covers a wider range of the estimates, from 5-95%.

A group of 22 top scientists used new techniques to measure how the ice will melt in the polar regions – which although rated as relatively low possibility, would have high-impact consequences.

Their study was published on Monday in the Proceedings of the National Academy of Sciences journal.

The researchers said that if emissions continue on the current trajectory then the rise would be between 62cm and 238cm by 2100, the result of an increase in world temperature of 5°C - the worst-case scenarios for global warming.

In this scenario, a number of coastal cities – including Los Angeles, London and Rio de Janiero - could be fully or partly underwater by the end of the century if climate change continues at the current rate, mostly because of the polar melt.

Study lead author Jonathan Bamber, from the University of Bristol, said the rising seas could result in a land loss of almost 1.8 million square kilometres, equivalent to three times the area of California.

"When you start to look at these lower likelihood but still plausible values, then the experts believe that there is a small but statistically significant probability that West Antarctica will transition to a very unstable state and parts of East Antarctica will start contributing as well," said Prof. Bamber.

"But it's only at these higher probabilities for 5°C that we see those type of behaviours kicking in."

The implications of such a rise in water levels go beyond land mass alone: it would displace some 187 million people and wipe out "critical regions of food production”, the study said.

Referring to the new study, Prof. Bamber said it's "important to consider the relatively low probability – but high-impact consequences – of ice sheet melt in the future."