Britain extended a ban on most non-emergency flights in its airspace by six hours to 1200 GMT Friday, due to ash from a volcanic eruption in Iceland, air traffic authorities said Thursday.

The National Air Traffic Services (NATS), which manages British airspace, said that "restrictions will remain in place in UK-controlled airspace until 1300 (UK time) tomorrow, Friday 16 April, at the earliest."

Some flights from Northern Ireland and the Western Isles of Scotland between some Scottish cities may be allowed between 0000 GMT and 1200 GMT, added NATS in a statement.

Services over the North Atlantic may also be allowed between some Scottish cities and Belfast, said the statement.

Although not visible on the ground, volcano ash can clog airplane engines as well as reduce pilots' visibility.

Scottish airports were the first to ground flights following Wednesday's eruption of the volcano under Iceland's Eyjafjallajokull glacier.

English airports followed before they were all closed in a virtually unprecedented move at 1100 GMT.

Cancellation of services at London's Gatwick and Heathrow airports - which normally handle almost 2,000 arrivals and departures a day between them - disrupted flights worldwide.

The two airports routinely deal with about 260,000 passengers a day.

Prime Minister Gordon Brown defended the decision of the air authorities to ban flights, stressing safety was paramount.

"The safety of our population is of the highest importance," he said.

"The aviation authorities have taken the temporary decision to suspend flights across (Britain) as this ash cloud moves over our country."

The volcanic ash has spread across northern Europe, cancelling thousands of flights and closing airspace in Belgium, Denmark, the Netherlands, Norway and Sweden.

Half of all flights between North America and Europe are expected to be cancelled on Friday due to the ash, according to Eurocontrol, the agency that coordinates flights in Europe.

There are an average of 600 flights between Europe and North America a day, it added.