The fire caused £10m of damage to the 19th Century ship
A fire which caused £10m damage to the Cutty Sark was sparked by an industrial vacuum cleaner that was left switched on for two days.
Police said the vacuum did not have a vital cut-off switch that prevents overheating because it had been adapted for a lower UK power voltage.
The fire caused serious damage to the 19th Century ship, based in Greenwich, south-east London, in May last year.
Conservation work was taking place on the vessel at the time.
The fire damage raised the total cost of the ongoing restoration by £10m to £35m.
The vacuum cleaner was being used to remove waste from the ship as part of a renovation work.
Investigators said it had been left on throughout the weekend before the fire broke out in the early hours of Monday 21 May, 2007.
More than 40 firefighters battled the blaze for almost two hours as it burned through each of the ship's three decks, destroying all the building work structures and tools on board.
Millions of pounds of damage was caused
The damage could have been far worse, however, as the ship's masts, deckhouses and saloon, along with half of its planking, had been taken to Chatham Historic Dockyard in Kent for conservation or storage.
Renovation of the ship is now expected to be completed by early 2010 when it will be raised 3m (9.8ft) above the bottom of its dry berth and suspended so visitors can walk underneath.
The vacuum cleaner had been left on for two days
Richard Doughty, chief executive of The Cutty Sark Trust, said the charity would "certainly be considering our position with our lawyers" following the report.
"Obviously the fire was a huge setback to the conservation project. We would have been on the programme to complete our work this year.
"Inevitably, therefore, the costs of our project have increased very significantly."
Mr Doughty praised the efforts of the fire brigade and subsequent police investigation into the cause of the blaze.
"We are very grateful to the police and to the fire brigade and would like to thank them for their diligence."
Detective Chief Inspector Dave Garwood, who led the inquiry, said two security guards who failed to spot the fire could have reported it sooner and a fire marshal inspection before the weekend could have helped prevent it.
Mr Garwood said the guards, who were both fired after the incident, were "vague and inconsistent" witnesses.
He also said that renovation workmen were responsible for dangerous practices onboard, including electrical equipment often left plugged in, debris not removed immediately and loose electrical connections.
It was also not clear if fire alarm tests were completed properly in the weeks before the blaze.
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