Dean Greenhill, 35, has suffered seven weeks of agony since he was bitten while working in south London.
He was left unable to walk and has needed two operations to drain pus and poison from his thigh.
The dad of two said: “I think we disturbed them while working on an old building and one got in my trousers.
“I was up on a roof and felt a sharp pain in my left leg. I dropped my trousers quickly and could feel the poison going up my leg.
“Soon after I was having trouble walking and the bite had swelled to the size of a tennis ball.
After missing weeks of work, Dean, of Ware, Herts, says he and his workmates are terrified. Worse still, on his first day back Dean found another of the spiders, a cousin of the deadly black widow, at a different site.
He said: “I kept clear this time. They’re definitely something people should be on the lookout for.”