The BBC's Lee Carter: "The Federal Aviation Administration say Mylar should be removed"

The Canadian Transportation Safety Board said metalised Mylar - an insulating material used in the aircraft to protect it from cold and noise - was flammable, and had helped spread the fire which brought the plane down.

The jet, an MD-11, was en route from New York to Geneva when it plunged into the Atlantic Ocean off the coast of Nova Scotia on 2 September, 1998.

The pilots had reported an unusual smell and smoke in the cockpit just minutes before they issued an emergency distress call to Canadian air-traffic controllers.

The investigators - who spent the last 11 months piecing together the final moments of Flight 111 - said cockpit insulation retrieved from the ocean floor revealed a significant amount of fire damage.

"While the source of the ignition has yet to be determined, there are indications that a significant source of the combustible materials that sustained the fire was thermal acoustical insulation blanket material," the safety board said.

The agency urgently recommended that all insulation blankets be tested for compliance with more rigorous fire standards.

Mylar failed tests

Soon after the Canadian announcement, the Federal Aviation Administration (FAA) in Washington said it would order US airline operators to replace metalised Mylar in 699 aircraft within the next four years.

Flight 111 disappeared from radar screens off Nova Scotia

But the FAA said it was "strongly encouraging operators to accomplish the insulation replacement during the earliest practical maintenance check".

It found Mylar fell far below fire resistance standards in a new test developed by the FAA.

FAA administrator Jane Garvey said: "We've weighed the benefit of replacing insulation, reviewed the service history of these aircraft and have made the right decision based on scientific data."

The planes involved were all made by McDonnell Douglas, now owned by Boeing. In addition to the MD-11, the aircraft affected are the DC-10, MD-80, MD-88 and MD-90.

McDonnell Douglas had recommended in 1997 that the insulation blankets be replaced on at least 1,000 jets.

Costly replacement

More than half of the affected aircraft are operated by US carriers, including American Airlines, Delta Airlines, Continental Airlines and Trans World Airlines.

Another 531 of the affected aircraft are registered in other countries.

The FAA estimated the cost of the insulation replacement to the US operators at about $255m, or between $380,000 and $880,000 per plane. Airlines said the true cost had not been determined.