Research by housing charity Shelter also found cases of families being harassed or illegally evicted by property-owners.
There are 3.5 million households in private rented accommodation, around eight million people.
While most landlords are honest and responsible, Shelter says local authorities must clamp down on a "small but dangerous group" whose actions are making people's lives a living hell.
Last week, Shelter named and shamed Manchester-based landlord Tame Estates which it accused of imposing rip-off charges and failing to carry out basic property repairs for tenants such as Chelsea Williams.
She was 16 when she moved from her foster home into a two-bed Tame Estates home in Ashton-under-Lyne, Greater Manchester, at the start of this year with her partner Grant and 13-month-old baby Harvey.
The property was in a state of disrepair when they arrived but the landlord promised to fix problems such as gas fires which were falling off walls and a broken window in the living room.
Eight months on, the repairs have yet to be done and more problems are emerging. The family are now squeezed into one bedroom because the other is too damp to use.
"My son can't settle into his room because the damp is so bad," says Chelsea.
"He's developed asthma and is always ill with colds."
The back garden is unusable because a blockedgrate means it is permanently flooded with dirty water from the sink and washing machine.
"This is my home and they're doing nothing about it. I need to move out of here, it's no place for a child to grow up," she says.
However Michael Hanley, boss of Tame Estates, denied Shelter's allegations insisting that he takes his responsibilities as a landlord "very seriously".
"With 220 properties and after 15 years in business there have only been a handful of court cases against me. The court has not upheld a single one of those complaints.
"I welcome Shelter's campaign to tackle rogue landlords but I am not one." A Ta m e s i d e Council spokesman said: "We estimate that Tame Estate holds about 2% of the private rented stock in the borough, but are the subject of about 15% of the complaints received this year."
Shelter say Chelsea's experience is all too common despite government claims that the system offers tenants protection.
It surveyed environmental health officers and more than 90% said they had encountered landlords who repeatedly ignore their responsibilities while a similar number said they had seen cases of severe damp, mould or fire hazards at properties they investigated in the past year.
Campbell Robb, chief executive of Shelter, said: "It is simply not acceptable that people are handing over their hard-earned cash to live in houses that are run-down, squalid and in some cases even dangerous.
"Our investigation shows just how ruthless a minority of rogue landlords can be. We know there are people operating in cities up and down the country like this and it's clear this is a national problem.
John Healey, Labour's shadow housing minister, attacked the ConDems for scrapping plans for tighter regulation of private landlords and letting agents.
"This would have given the most vulnerable in the private rented sector some real safeguards," he said.
Shelter also highlighted some common scams landlords used to exploit the vulnerable .
They include breaking into empty properties then renting them to unsuspecting tenants who hand over money as a deposit and the first month's rent, at which point the crooks disappear.
Others impose hidden, often sky-high charges for things such as letters and property inspections immediately putting people in arrears.
Rather than asking for a deposit, the landlord requests tenants provide guarantors. At the end of their tenancy these guarantors can become liable for unnecessary and extremely costly 'repairs'.
Although it is a legal requirement, some rogue landlords still avoid putting tenant's deposits in a tenancy deposit scheme, withholding it at the end for unfair reasons.
David Salusbury, chairman of the National Landlords Association, insisted that given there were nearly 3.5 million households in private rented homes, the number of cases that turned sour was relatively small.
"Some of the scams mentioned by Shelter have nothing to do with landlords but are the acts of professional criminals - we roundly condemn such practices."
TIPS TO ESCAPE THEIR CLUTCHES
1) Carefully check the type of tenancy agreement you are signing.
2) Find out if the building requires a licence.
This is likely if it is a house of multiple occupancy such as a house split into bedsits.
3) Check how your deposit will be protected.
4) Make sure the inventory is accurate.
5) Set up a standing order to be sure your rent is paid on time. 6) Stick to the rules eg no smoking or pets.
7) Check safety of gas appliances.
8) If repairs are needed check whose responsibility it is.
9) Make sure repairs are done by registered workmen.
10) For further advice log on to Shelter's Evict a Rogue Landlord campaign.
By Clinton Manning 15/09/2010