Aerospace giant Boeing on Wednesday halted test flights on its new 787 Dreamliner, dealing a fresh setback to a program already running more than two years behind schedule.

Boeing announced the decision after a fire aboard a test plane on Tuesday had forced an emergency landing.

At a news conference in Seattle, Wash., home of Boeing's commercial airplane factory, spokeswoman Loretta Gunter said the fire was the most serious incident since the test flight program began on Dec. 15, 2009.

"We really don't know" whether the suspension of test flights will further set back the program, she said. "I don't know how long the suspension will last."

The 787 program, launched in April 2004, has suffered a series of delays, many of them from challenges in the international production of parts for the midsize plane made essentially from composite materials.

Boeing says the high-tech 787 will deliver a 20% reduction in fuel consumption compared with planes of similar size flying today.

Gunter said the Chicago-based company would focus on ground testing "and not fly the airplanes until we better understand the incident on ZA002."

"The most likely outcome is a modest delay to the flight test with a potential multiweek slip in first delivery, which is well within the range of investor expectations," Barclays Capital analysts said in a client note.

Smoke filled the cabin of the ZA002, one of Boeing's six test 787s, on Tuesday, forcing an emergency landing in Laredo, Texas.

"There was a fire on board the airplane, which created the smoke in the cabin area," Gunter said.

She stressed the investigation was of "an incident, not an accident."

"We don't know where it started. We need to analyze all the data," she said, adding the fire had not been in the main cabin.

It was extinguished "a very short time before landing after several hours of flying," she said.

The troubled plane was painted in the livery of Japanese launch customer All Nippon Airways (ANA) and, like three other 787 test planes, was equipped with a Rolls-Royce Trent 1000 engine. The remaining two have General Electric engines.

"Right now we don't see any connection with the engine," Gunter said.

The British firm has been in the spotlight after a spate of midair mishaps.

In early November, a Qantas Airbus A380 superjumbo was forced to make an emergency landing after a Rolls-Royce Trent 900 blew out.

In a separate incident, a Qantas Boeing 747 had to turn back to Singapore after another model of Rolls-Royce engine failed in midair.

In August, Boeing pushed back the 787 delivery schedule from a target of the early weeks of 2011 due to a delay in the availability of a Rolls-Royce engine.

Gunter said a Roll-Royce engine had exploded during ground tests in September.

It was the second time Boeing has halted 787 test flights, suspending them for "a couple of days" in June, she said, without explaining the reason. At the time, U.S. media reported it was because of problems with the plane's tail stabilizers.

According to the company's latest schedule, the first 787 will be delivered to ANA "in the middle" of the 2011 first quarter -- around February. The initial plan was for delivery in the first half of 2008.

Last week Aviation Week reported that Boeing has warned some 787 Asian customers of delays of up to 10 months, citing industry sources.

The airlines affected were Korean Air, Air India and Japan Airlines, the industry publication said.

Copyright Agence France-Presse, 2010