Last Saturday the same volcano triggered a tsunami, which killed hundreds of people.

Air traffic control agency AirNav Indonesia said it was closing flight routes because the volcanic ash Krakatau was spewing ash, which meant the situation was on "red alert".

An AirNav operations manager told the BBC between 20 and 25 flights were affected, including some international flights to and from Australia, Singapore and the Middle East.

Disruption is likely to be minimal, he added, although passengers may experience longer journeys and aircraft may need more fuel due to the diversions.

Indonesia’s disaster management agency said people and tourists were prohibited from carrying out activities within a 5km radius, “as the volcanic activity of Anak Krakatau continues to increase”.

The UK Foreign Office is continuing to advise those travelling to coastal areas of Western Java and the South of Sumatra, close to Mount Anak Krakatu volcano, to check with their travel agents, monitor local media and follow the advice of local authorities.