A raid on a three-bedroom house in north west London found 35 men living in rooms full of mattresses.
The discovery was made on Winchester Avenue, Queensbury, on Tuesday following noise complaints from neighbours, Brent council said.
The men, of Eastern European origin, had piled bedding in every room except bathrooms.
The owner of the property is being traced.
The council said it was a “shocking example” of a rogue landlord “exploiting people”.
The council’s raid on the semi-detached property found dozens of shoes piled on top of each other, clothes strewn across radiators and junk clogging the garden. One mattress was laid out under a canopy in the back garden.
Harbi Farah, Brent’s cabinet member for housing and welfare reform, said it highlighted the worst excesses of unscrupulous property owners.
“Rogue landlords make their money by exploiting people who can least afford it, it’s a shameful practice and this is an especially shocking example”, he said.
In just one room, eight men were found to be sleeping side-by-side on “wall-to-wall” mattresses.
Mr Farah added: “Any landlord treating their tenants unfairly should be on notice: we’re coming for you.”