Scientists are “very worried” that the melting of the Greenland ice sheet could accelerate and raise sea levels more than expected.

They say warmer conditions are encouraging algae to grow and darken the surface.

Dark ice absorbs more solar radiation than clean white ice so warms up and melts more rapidly.

Currently the Greenland ice sheet is adding up to 1mm a year to the rise in the global average level of the oceans.

It is the largest mass of ice in the northern hemisphere covering an area about seven times the size of the United Kingdom and reaching up to 3km (2 miles) in thickness.

This means that the average sea level would rise around the world by about seven metres, more than 20ft, if it all melted.

That is why Greenland, though remote, is a focus of research which has direct relevance to major coastal cities as far apart as Miami, London and Shanghai and low-lying areas in Bangladesh and parts of Britain.

Algae were first observed on the Greenland ice sheet more than a century ago but until recently its potential impact was ignored. Only in the last few years have researchers have started to explore how the microscopically small plants could affect future melting.

A five-year UK research project known as Black and Bloom is under way to investigate the different species of algae and how they might spread, and then to use this knowledge to improve computer projections of future sea level rise.

The possibility of biologically inspired melting was not included in the estimates for sea level rise published by the UN’s climate panel, the IPCC, in its latest report in 2013.