Landlords are earning £5.6bn a year by renting out unsafe homes which fail to meet legal standards, a report says.
The Citizens Advice study says 740,000 households in England live in privately rented homes which present a severe threat to tenants' health from problems like severe damp and rat infestations.
Gillian Guy, chief executive of Citizens Advice, said: "Rogue landlords are putting profits before safety."
The government said it had given councils new powers to tackle them.
The report, A Nation of Renters, says that among these 740,000 households are 510,000 children while 180,000 of them have a disabled resident.
And it says landlords are receiving £5.6bn a year on rent for homes with category 1 hazards - the most severe - which includes £1.3bn in housing benefit.
The report also says:
- 16% of privately rented homes are physically unsafe - compared with 6% in the social rented market
- 8% of privately rented homes have serious damp
- 10% pose a risk of a dangerous fall
- 6% are excessively cold
- Private renters living in homes with a category 1 hazard pay an average of £157 per week on rent
Ms Guy added: "The government has rightly said it wants to tackle the country's housing crisis - it must make targeting dodgy landlords, giving tenants better rights and driving up standards a major part of that effort."
Citizens Advice says there are now more than a million families raising children in privately rented homes in England - three times higher than a decade ago.
It also says private renters are under-protected and that taking court action against a landlord can be a lengthy, complicated and expensive process.
The charity recommends that:
- Tenants should be entitled to rent refunds where properties are dangerous or not fit to live in
- A national landlord register should be set up to help ensure landlords operating illegally cannot move to different areas to avoid legal action
- Councils should also set up local licensing to tackle specific issues in their private rental markets
The report is part of Citizen Advice's Settled and Safe campaign.
In the last year more than 80,000 people asked the charity for advice regarding a problem with a privately rented home.
The campaign called for an end to retaliatory evictions - where landlords unfairly evict tenants who have raised problems - which will be made illegal later this year.
Housing Minister Brandon Lewis said: "No tenant should expect to live in unsafe housing which is why we have already introduced a range of powers for councils to tackle rogue landlords backed by £6.7 million of government funding.
"Tenants can also access our 'How to Rent' guide and model tenancy agreement so tenants and landlords alike are clear of their rights and responsibilities."
And he added: "The majority of landlords provide decent well maintained homes and unnecessary and excessive regulation on the private rented sector, would push up prices and restrict choice for tenants."