A massive fire which destroyed much of the National Motorcycle Museum near Birmingham, ruining 650 vintage machines, was caused by a "carelessly discarded" cigarette, investigators say.
Three of the five exhibition halls were destroyed
The value of the destroyed bikes is put at £8m, but many of the machines are irreplaceable.
The destruction could have been worse - scores of bikes were pushed to safety by museum staff and delegates at a business conference on the city.
Fire experts at the scene on Wednesday quickly determined the cause of the fire which has left 70% of the building a blackened shell.
History of motorcycling
Speaking from the scene to BBC Midlands Today, West Midlands Fire Service spokesman Dave Glenis said the fire started outside the building in boxes of old air conditioning units.
"It appears that this is very close to the area where people smoke.
"It was the careless disposal of a cigarette into those boxes. That's why the fire spread so rapidly through the building.
"We are not treating the incident as arson at this stage and
believe it was purely an accident," he said.
More than 120 firefighters tackled the fire which could be seen for 15 miles
The bikes destroyed in the collection traced the history of the British
motorcycle industry and many of the museum's rarest exhibits were among those lost.
Exhibits included BSA Gold Stars, Sunbeam Twins, DMW Hornets and Triumph racers.
But museum bosses have vowed that the museum will be rebuilt and hopes remain that some of the damaged bikes could be saved.
Neil Payne, competition and demonstrations manager at the museum, said: "I am sure that it will rise, like the proverbial phoenix, from the ashes.
"At the moment we do not know whether the bikes are destroyed beyond redemption.
"We keep thinking it's a bad dream and we are going to wake up.
"This is a huge loss to British heritage."
Staff at the museum were said to be "absolutely devastated".
A museum spokesman said: "Over 300 of the exhibits in the museum have been saved.
"Sadly, three of the museum's five exhibit halls have been destroyed along
with their exhibits.
"A full assessment of the damage is being undertaken with a view to getting
operations under way again as soon as possible.
"Many irreplaceable machines will have been damaged in the fire, but they
will be restored once again, to their original showroom condition."
The museum was home to more than 900 vintage machines.
Owner Roy Richards started collecting the bikes in the 1970s and the museum opened in 1984 with an initial collection of 350 machines.
It became one of the biggest motorcycle museums in the world and attracted about 250,000 visitors a year. It was also a conference centre.
Mr Richards said: "It is a devastating loss, not just to me, but for the country as a whole."
Neil Payne told the BBC: "There are so many unique, priceless, irreplaceable machines that are lost. It is absolutely devastating.
"The museum and indeed the country has lost some unique machines from British motorcycling history."