David Myers, 72, who has been in the antiques business since he was 18, said the two top floors of the five-storey building have been destroyed and the stock of ten dealers has been lost.
The fire started at around 11.30am in room 28 on the fifth floor of the 82-room centre.
No one was in the room at the time, but there were five staff and about 30 members of the public in the building when the alarm sounded. Three staff members tried to fight the blaze before the first fire engine was on site with five other appliances arriving shortly afterwards.
"The firemen were brilliant," said Myers. "They were in there covering stuff up to stop the water damage, but some of it couldn't be saved. The top two floors have been written-off." Six to eight rooms at the top of the building have been completely destroyed and extensive repair work is needed to the roof.
The damage to the rest of the building and to the stock of 70 dealers has been caused by water. It was necessary to empty every room to allow fire investigators and insurers full access to the building.
Appeal for Dealers
"We started work the following morning getting everything out and trying to dry out the millions of gallons of water used to put out the fire," said Myers.
"We have no power but [my sons Paul and Ben] organised generators to provide lighting and arranged for the dealers to come in to get all the stock out."
An appeal has been launched on JustGiving.com to raise money to help the ten traders who lost everything.
"Everywhere will need recarpeting, rewiring and painting, but we hope to have half the building open as soon as is humanly possible," added Myers.
"I reckon it will take three or four months to rebuild. The mill is a much-loved building. We've had calls from Australia, Antigua, Holland and Germany from well-wishers and lots of support on Facebook and Twitter. There'll be one hell of a party when we reopen."
Ahead of the clean-up operation, Myers has consigned family furniture rescued from the blaze to Ewbank's. The catalogue for the Surrey auctioneers' final sale of the year on December 16 was reopened specially and the 40-plus lots will be sold without reserve.