Heathrow airport shut by ash cloud (ABC News)

The spreading ash has already forced the closure of most of Britain's northern air space. (ABC News ...

London's Heathrow airport will be among more airports closed for six hours from 1:00am (local time) on Monday as ash from an Icelandic volcano drifts further south.

"The high density ash cloud continues to move further south in the early hours" of Monday, said a statement from the National Air Traffic Services (NATS), which manages British airspace.

The statement said the no-fly zone, including London Heathrow and Gatwick, would be imposed from 1:00am until 7:00am Monday.

The spreading ash has already forced the closure of most of Britain's northern air space.

Qantas says the closures will not affect its flights but it is monitoring the situation.

The high-density ash cloud drifting south from Iceland means restrictions are now in place over the Midlands, northern England, Scotland, Wales and Northern Ireland.

Much of Europe's airspace was closed for six days last month over fears that ash from the Icelandic volcano would cause aircraft to crash.

According to the International Air Transport Association airlines lost $2 billion.