LONDON — Civil aviation authorities said today that flights won't start departing or arriving in Britain until at least Friday morning because of a huge ash cloud spewed by an Icelandic volcano.

The National Air Traffic Service said today that nonemergency flights won't be permitted in British airspace until at least 1 a.m. Friday.

A statement from the authority said the "cloud of volcanic ash is now spread across the U.K. and continuing to travel south."

The Icelandic volcano was sending up smoke and ash that posed "a significant safety threat to aircraft," as visibility is compromised and debris can get sucked into airplane engines.