A housing charity investigation in to private landlords shows complaints to councils about poor housing have jumped 27% in the past three years.
Shelter wrote to every council in England and received responses from 310, who said they had around 85,000 complaints from private tenants.
Around 62% were about dangerous gas and electricity supplies and severe damp.
Councils reported concerns about 1449 private landlords, with 49 councils calling in health services to deal with 781 cases relating to the behaviour or neglect of private landlords.
From 2010/11 to 2011/12, the number of successful prosecutions against private landlords increased by 77%.
During 2011/12 there were 487 successful prosecutions out of 596 private landlords taken to court.
The most prosecutions come from councils in Newham, East London; Leeds; Salford and Manchester.
Shelter chief executive Campbell Robb said: “Despite the significant increase in complaints, we believe that the number of rogue landlords is still underestimated.
“Some local authorities don’t keep records of complaints and tenants often hold back from complaining out of fear of the consequences or because they don’t believe their voices will be heard, even though such a high proportion of complaints is about life-threatening issues.
“Every day at Shelter we see the devastating impact these landlords have on people’s health and well-being. There could be thousands more victims of these operators, trapped in homes that cause misery and, in some cases, put lives at risk.
“It’s ultimately local authorities that must do everything in their power to support people who are suffering by cracking down on the worst offenders in their area.”
Although the number of complaints is increasing, the number of private landlords owning buy to let properties has also risen during the same period to 3.9 million.
Shelter has no figures on the ratio of complaints from tenants to the number of buy to lets in 2008/09 to the higher number of properties now.