"The building has taken a large amount of damage, we have not been able to get into the property due to the fire and the structural integrity," said Mr Webster.
He added the "very large substantial fire" had been "within a warehouse itself".
People living nearby have been advised to keep their windows and doors closed due to the smoke.
James Golder, who works at nearby Altro Walls, said he saw the flames and a plume of smoke fter coming out of the 24-hour gym and could still see it when he returned home.
He said: "I noticed it was the Britannia building which is next door to ours… when I got closer that's when I could see how high the orange flames were.
"I can only imagine what it felt like for any staff at Britannia… you never think it's going to be your building… it's frightening.
"God forbid if it was during the day… it's just fortunate there was no one in at the time it makes you consider everything, you don't expect it to be the building next to you."
