Drug-dealing, violence and anti-social behaviour are rife at a converted office block likened to a "human warehouse", an investigation has found.
Homeless families sent by councils to flats in Templefields House, in Harlow, Essex, ended up living there alongside ex-prisoners and drug addicts.
The BBC East and Panorama investigation found evidence security staff had lost "control" of the building.
Landlord Caridon said all criminal behaviour was dealt with "robustly".
"As a result of the specific allegations made by Panorama we have commenced our disciplinary procedures to investigate alleged security and privacy breaches," it said in a statement.
The investigation revealed police had received almost 600 calls from the building in three years.
Three police raids took place during the three weeks of filming and four people were subsequently convicted of supplying Class A and Class B drugs.
Staff said they were unable to control what goes on inside. The 172 flats, converted in 2017, house ex-prisoners, people suffering mental health problems and people with drug issues, alongside victims of domestic violence and young families.
A security guard told an undercover BBC reporter that drugs were a serious problem.
"A lot of them are dealing from there. You can't control 172 flats," he said.
A resident was filmed describing how she had been offered drugs by individuals hanging around outside the building.
Undercover footage also shows anti-social behaviour and fights inside.
Staff told the undercover reporter about a "welfare check" they said they were asked to do by a police officer. During the visit, a diary was found and photographs were taken of it.
The staff discussed private details from within it, which, they said, was shared with a police officer.
Panorama approached the two members of staff involved but they did not respond.
Essex Police said it had "no knowledge of any diary".
Nic, a resident who has lived there for more than three years with his wife, described the living conditions as "human warehousing".
He said his terminally ill wife had been threatened by a fellow resident.
"Some guy tried to pick a physical fight with her two months ago," he said.
"He snaps at her and launches a verbal and physical attack. She's not happy to go outside by herself."
Harlow Council has started removing its tenants from the building.
Leader Mark Ingall, of the Labour Party, said: "It's appalling. In the fifth-richest country in the world, we should be able to house people and meet decent standards of housing.
"These office block conversions are housing on the cheap."
The Liberal Democrat leader of Three Rivers District Council in Hertfordshire, Sara Bedford, apologised for sending tenants - including young children - there.
She said: "I am so glad personally that we are able to get our tenants out... I am sorry that we ever sent people there in the first place."
One mother sent by Three Rivers, Kasey Thompson, told how she felt "constantly on edge".
"There were people who were quite angry because they've been in and out of prison, there's people who quite literally had fights outside the front door, you hear them kicking and screaming," she said,
She added: "I was under the impression I'd be with mums and kids, and then I'm with drug addicts, in flats with burglars, all under one roof. How am I supposed to feel safe?"
Caridon said local authorities occasionally referred families to them and they had "no say" as to whether we accept or reject such tenants.
"We have a zero-tolerance approach to criminal and anti-social behaviour on all of our sites and act robustly, including evicting tenants where necessary," it said.
"We have a strong and close working relationship with the authorities, local agencies and the police."