The risk level for Iceland’s Bardarbunga volcano has been moved up to level four on a five-grade scale for danger of eruption.
Ash from the eruption of Iceland's Eyjafjallajokull volcano in 2010 shut down much of Europe's airspace for six days, affecting more than 10 million people and cost European businesses up to €2.5 billion.
Officials have been monitoring volcanic activity at Bardarbunga, Iceland’s largest volcano, since 16 August, when the Met Office measured the strongest earthquake in the region since 1996.
There are strong indications of ongoing magma movement although no signs of eruption yet.
Bardarbunga is located under the ice cap of the Vatnajokull glacier in the southwest of Iceland.
It is in a different range to Eyjafjallajokull.
Met Office seismologist Martin Hensch said the risk of any disruptive ash cloud similar to the one in 2010 would depend on how high any ash would be thrown, how much there would be and how fine-grained it would be.
"Presently there are no signs of eruption, but it cannot be excluded that the current activity will result in an explosive glacial eruption, leading to an outburst flood and ash emission."
Mr Hensch said the biggest risk in Iceland itself was from flood waves from any eruption under the glacier.
He said the area of Iceland mainly at risk of flooding was mostly uninhabited but that roads in the area had been closed.