One in nine privately-rented homes in England has had problems with vermin such as rats, mice or cockroaches in the last year, research from Shelter suggests.
A survey for the housing charity found that 11% of privately-rented homes have had vermin problems in the last year - equating to almost half a million homes, if the findings were projected across England.
Almost half (49%) of tenants surveyed said they had experienced poor conditions or disrepair in the last 12 months.
More than one third (34%) of private renters said they had lived in a property with damp or mould and over a fifth (22%) said they had suffered due to poor insulation.
A further 11% of renters also reported electrical hazards, the survey of more than 3,700 private renters in England found.
Campbell Robb, Shelter's chief executive, said more action is needed to protect renters living in "shoddy properties".
Shelter helpline adviser Nadeem Khan said: "Every day at Shelter we speak to people desperate for help, after being forced to live in appalling conditions for months on end.
"Some of the housing horrors we hear are truly shocking - from people who are forced to live in flats crawling with fleas or rats, or with water pouring out of the toilet waste pipe, to those who've been through the ordeal of a fire in their home caused by faulty wiring."
In one case seen by Shelter, a mother reported having to live with exposed wires and "rats running everywhere".
A spokesman for the Department for Communities and Local Government said that the vast majority of landlords and letting agents provide a good quality service in the private rented sector.
He continued: "However, the Government is determined to crack down on rogue landlords and the Housing Bill strengthens councils' powers to tackle poor-quality privately rented homes in their area.
"We have also introduced a range of measures so tenants and landlords alike know their rights and responsibilities."
Measures included in the Housing Bill include creating a database of rogue landlords and letting agents, to help councils to focus their enforcement action on where it is most needed and keep track of those who have been convicted of housing offences.
The Government has also introduced new guidance on revenge evictions so that a landlord cannot evict a tenant for making a complaint regarding the condition of the property.
Here are Mr Khan's tips for tenants on dealing with poor conditions:
1. Be clear on who has responsibility for what. Your landlord is responsible for dealing with most repairs in your home, including problems with the roof, walls, windows, central heating and drains. They must keep the water, gas and electricity equipment safe. As a tenant you will often have responsibility for minor repairs and maintenance, which can include small jobs like replacing lightbulbs.
2. Tell your landlord if you have a problem with pests or vermin. They should deal with problems that were already there when you first moved in. If you caused the problem, by not clearing away leftover food or rubbish, you will probably have to sort it out yourself. See www.shelter.org.uk/advice for more help.
3. You can help prevent problems with mould by heating your home and keeping it properly aired. If it is a constant problem, tell your landlord. Major repairs, such as leaking pipes or rising damp, are your landlord's responsibility. If they will not help, you can call Shelter's free helpline on 0808 800 4444 to speak to an adviser.
4. Faulty electrics are very dangerous so report any concerns to your landlord straight away. Your landlord should make sure the electrics are safe. If they do not do this, you can complain to the environmental health team at your local council.