Story Highlights • Flames consume London's historic clipper ship Cutty Sark

• Vessel was undergoing $50 million restoration at time of blaze

• Cause of fire under investigation

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LONDON, England (CNN) -- The Cutty Sark, thought to be the world's only surviving 19th century tea clipper and a prime relic of the golden age of sail, was engulfed by flames Monday, causing extensive damage to one of London's most popular tourist attractions.

Police say the blaze, which began before dawn and took 40 firefighters several hours to extinguish, is being treated as "suspicious." The fire is expected to drastically increase the cost of a $50 million three-year restoration that began in 2006.

Witnesses described flames leaping 20 feet (six meters) in the air, fanned by strong winds. Nearby residents were temporarily evacuated. (Watch flames sweep through historic ship )

The clipper -- a very fast multi-masted sailing ship used for transporting high value goods such as tea and wool -- was once regarded as the pinnacle of merchant sail vessel development.

Standing by the smoldering hull of the vessel, Chris Livett, chairman of Cutty Sark Enterprises, told CNN that although the ship's decks were "unsalvageable", he believed the vessel could be fully restored.

He said half of the timbers and all historic artifacts on board had been removed for the conservation project.

"I do not think we can give a figure on how much this damage has cost," he told CNN. "We're in dire straits now."

He added: "This ship has been through many things in its life, it's sailed through the oceans and been through many natural disasters and survived.

Speed under sail

"We're devastated. Lots of people on this project worked very hard and to see that effort go up in smoke is very hard."

More than 15 million people have visited the ship in its dry dock at Greenwich, a World Heritage Site on the southern bank of the River Thames. The vessel has been on display since the 1950s.

Cutty Sark rose to prominence after leaving London on its first voyage on February 16, 1870, and sailing to Shanghai. Capable of reaching speeds of up to 17.5 knots and covering more than 360 miles a day, it outstripped most other vessels. (Read profile of proud maritime relic)

In 1885, the 85-meter (280 feet) ship achieved a record-breaking wind-powered voyage from Australia to England, completing the distance in 72 days. But its glory days were short-lived as steam ships quickly came to replace sail.

Chief executive of the Cutty Sark Trust, Richard Doughty, described the damage as extensive, but said he was hopeful that crucial infrastructure had survived the fire.

"At the moment we are going to try to assess what damage has been done to the metal frame. A cursory glance indicates that may have survived the fire," he told CNN.

Doughty paid tribute to the vessel's status and expressed sorrow over the blaze.

"It is the epitome of speed under sail. She was a legend in her working lifetime, she set all sorts of records. Above all, for Londoners, she's an icon.

"Personally, I'm gutted. The smell of the ship, which is pervading over the whole area, the tar of the rope and timbers, usually you can only smell that on the ship, but to smell that all around brings sadness."