Mohammed Kamel Mostafa, son of radical Muslim cleric Abu Hamza al-Masri, was identified by colleagues who saw his picture in the newspaper, The Sun reported. Mostafa was working as a laborer for a contractor.
A source told the newspaper that Mostafa, while he had his job, was able to go into areas off limits to the general public.
"The fear was he could have planted anything down there and he had access to the top area," the source said. "It's the area that, if hit, would cause the maximum damage imaginable. I can't believe he slipped through the net."
A spokesman for the Underground said that Mostafa apparently got a permit because he has a clean record in Britain and is legally able to work.
Last year, four suicide bombers attacked London's mass transit system on July 7, causing explosions on three Underground trains and a bus.
Hamza was sentenced to seven years in prison earlier this year after being convicted of inciting terrorism.