Housing Justice has been reacting to the findings of a study which show as many as 1 in 6 privately rented houses are substandard.
Some have severe hazards like mould, damp, extreme cold and infestation of rats and insects.
Other properties are severely overcrowded with one police raid finding 17 men in a three bedroom north London house.
"It can be really bad," warned Housing Justice CEO Alison Gelder. "People with water running down the walls, black mould and horrendous overcrowding."
She said many landlords are not keeping up with work needed to keep their properties inhabitable: "There are a lot of landlords for whom it isn't their main job.
"People who have one or two properties as a kind of hobby to fund their pension are not necessarily as quick as they might be at doing repairs".
Local authorities warn Britain's housing crisis and a chronic lack of social housing have caused a huge increase in the number of people having to rent from private landlords.
High rent prices mean many poorer people are having to take substandard accommodation just so they can afford somewhere to live.
When asked if Britain was heading back to Victorian era inner city slums Alison Gelder replied: "I think we've been taking a step back for a while".
She said Housing Justice was offering help to landlords who want to do a good job but are new to the industry.
"There are a lot of Christian landlords out there who want to do a good job but it isn't always easy to find out what you should be doing and how you should be treating your tenants," she said.
Alison Gelder speaking to Premier's Alex Williams: