A fire that tore through a famous 19th century British ship was likely caused by a vacuum cleaner used for renovation work bursting into flames, police said Tuesday.

Investigators found no evidence of arson for last year's fire that threatened to destroy the Cutty Sark, a historic three-masted tea clipper that sits in dry dock on the banks of London's River Thames.

The most likely causes were a vacuum cleaner catching fire after it was accidentally left on over the weekend or electrical equipment that overheated, according to a report released Tuesday.

"It is the view of the inquiry that the cause of the fire was accidental," police inspector Dave Garwood told reporters.

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Firefighters battled the blaze for more than two hours early on May 21, 2007, near Britain's National Maritime Museum at Greenwich.

The inferno sent clouds of black smoke over the leafy southeast London borough, but much of the Cutty Sark's historic material — including its masts, deckhouses, and most of the ship's hull planking — had already been sent to the Chatham Dockyards, east of London, for conservation work.

The trust said less than 2 percent of the vessel's original material was lost to the flames.

"It could have been much, much worse," the Cutty Sark Trust's chief executive Richard Doughty said. "We came very close to losing the ship."

The Cutty Sark, which made its maiden voyage in 1870, is the world's only surviving example of an extreme clipper, regarded as the ultimate development of a merchant sailing vessel.

Designed to race its cargo of tea from the Far East back to Britain, the ship was restored and installed in a dry dock in 1954 in Greenwich, a historic naval center and World Heritage Site.

Given its importance, police said they interviewed more than 70 people during a 16-month probe and reviewed some 250 hours of surveillance footage.

"The investigation had to be made to the standard of a murder investigation," Garwood said.

The conservation program, originally intended to fix the ship's wrought iron frame, has been delayed by nearly two years, Doughty said.

He added that the fire added $18 million to the cost of the restoration — and that the trust was still about $5.4 million short of the funds needed.

The Cutty Sark is due to reopen to the public by the summer of 2010.