All British airports were closed on Thursday after a massive cloud of ash from an erupting volcano in Iceland drifted into Britain's airspace.

Smoke billows from a volcano in Eyjafjallajokull April 14, 2010. A volcanic eruption in Iceland spewed black smoke and white steam into the air on Wednesday and partly melted a glacier, setting off a major flood that threatened to damage roads and bridges. [Xinhua]

All flights in and out of Britain were suspended, disrupting about 300,000 passengers.

British airspace will be closed until at least 6 p.m., apart from emergencies. The closure includes military flights through Royal Airforce Brize Norton Oxfordshire, the main airbridge to Afghanistan.

The Air Traffic Control Service (NATS) said no flights would be allowed in or out Britain airspace amid fears of engine damage. The airspace restriction was the most significant in living memory, a NATS spokesman said.

The Civil Aviation Authority called the shutdown the biggest since World War II.

Airspace in Belgium, Germany, Ireland and Scandinavia was also closed -- affecting an estimated one million passengers across Europe.

British Airways said all of its flights were cancelled until Friday morning at the earliest.

Experts have warned that the tiny particles of rock, glass and sand contained in the ash cloud would be sufficient to jam aircraft engines.

The Health Protection Agency said, however, that the ash did not pose a significant risk to public health because of its high altitude.