Sometimes tenants come home to find the locks have been changed
Campaigners are urging the government to help protect thousands of private tenants facing eviction as landlords increasingly fall into arrears.
The Citizens Advice Bureau, Shelter, Crisis and the Chartered Institute of Housing have joined forces to highlight what they say is a growing problem.
They want new laws in England and Wales to give tenants more notice when their home is repossessed by mortgage firms.
Currently only two weeks' notice is required, leaving many homeless.
The government has so far said it will increase it to seven weeks, from 6 April.
Private sector tenants have few rights to protect them when their buy-to-let landlords get into trouble, say the campaigners.
A survey carried out by Crisis found that 60% of its advisers had been approached by people who had lost their home in this way, 80% of whom said the problem was increasing.
The groups said the tenants of these properties were the "forgotten victims of the repossession crisis".
The charities estimate that more than 8,000 buy-to-let properties could be repossessed in the coming year, with at least 10,000 people being made unexpectedly homeless.
In some cases families are given no warning at all, sometimes returning home to find locks had been changed and their possessions out on the street, they said.
In one instance a family had to spend the night sleeping in their car, before being moved into emergency hostel accommodation, when their rented home was repossessed.
The charities are calling for the law to be changed to allow courts to defer repossessions, enabling tenants to find new accommodation.
They have written to MPs urging them to sign an Early Day Motion calling for the law to be changed.
More also needed to be done to make tenants aware of repossession proceedings, such as getting the courts to send notices clearly marked "to the tenant" of a property, in addition to notices currently sent "to the occupier" by mortgage lenders.
Shelter chief executive Adam Sampson said they had seen a "steep rise" in cases.
"Tenants who have kept their side of the bargain by paying their rent are being thrown out on to the street because their landlords have defaulted on the mortgage."
Leslie Morphy, of Crisis, said with the economic troubles the focus was mainly on homeowners.
"We risk forgetting that tenants of private landlords are extremely vulnerable to the recession," he said.
The Council of Mortgage Lenders said many lenders would consider continuing a tenancy if a landlord got into difficulties, with the tenant paying their rent directly to the lender for the period of the tenancy.
But it said in some cases, landlords might be renting out a property that had an owner-occupier mortgage on it, without the lender's knowledge.
A spokesman for the Department of Communities and Local Government said they were "determined" to make sure tenants facing eviction through no fault of their own were protected.
Other than increasing the notice period to seven weeks, the department said it was looking into what more help could be provided for tenants, either through legislation or "influencing lender behaviour".
"In the first instance, it is vital that landlords struggling with their payments contact their lender," added the spokesman.