The files, many of which were stamped "confidential," date from the early 1980s to 1993 and one privacy expert calls it "a treasure trove for ID thieves," the Sun said Monday.
"All the data you need is there for ID theft or to clone someone else's identity," the expert said.
The council for the northern London borough of Haringey vacated the building in May 2005 and it's been used by "squatters" since then, the Sun said.
Lynne Featherstone, who represents the district in Parliament, said the council should let everyone know if they face a risk of ID theft.
Haringey Council members said they were investigating the matter.