A new cloud of volcanic ash was headed toward Britain on Tuesday, increasing the uncertainty about the prompt return of air traffic to parts of the British Isles, according to the organization that controls air traffic in the area.

“The volcano eruption in Iceland has strengthened, and a new ash cloud is spreading south and east toward the U.K.,” the British National Air Traffic Services, or NATS, said in a statement posted on its Web site early Tuesday. “This demonstrates the dynamic and rapidly changing conditions in which we are working.”

On Monday, European officials announced that all European airspace would be opened to air traffic above elevations of 20,000 feet.

Below 20,000 feet, NATS said on Monday afternoon, Scottish airspace might reopen Tuesday morning and that restrictions over England and Wales, including the London area, might be lifted later in the day. But on Tuesday the agency said flights were being permitted in Scotland and parts of England, but not as far south as the main London airports.