The Icelandic Met Office has raised the elevation colour code at Grimsvotn volcano from yellow to orange due to increased activity and the potential to affect flight routes.
It said seismic activity had been increasing above normal level for two days, with a magnitude 3.5 earthquake occurring just after 6am.
However, no volcanic tremor has been detected and no increases or changes in geochemical emissions measured. Grimsvotn last erupted in 2011. It is said to erupt roughly every 10 years.
The Foreign Office updated its travel advice on Monday (6 December) to reflect the developments.
"An orange warning has been issued for Grimsvotn volcano in south east Iceland," said the FCDO. "The likelihood of an eruption is increased. Further volcanic eruptions are possible."
