The deadly Brazilian Wandering spider was spotted beside damp wooden pallets from South America by Lee Hilton, a worker at shower and tap company Bristan, in Tamworth.
"I thought it was a wolf spider to start off with," he said.
Staff at Bristol Zoo found it to be a female, possibly the largest seen in the UK.
James Ship, an expert at Stratford Butterfly Farm, which also houses arachnids, said if bitten by the spider, "you would have about six hours' worth of pain".
He added: "If you weren't able to get anti-venom within those six hours, it can lead to paralysis and in some cases it can lead to death."
It is understood the spider is undergoing tests in London.
An exotic spider known as one of the most venomous in the world has been found in a Staffordshire warehouse.