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, at around 6am on Tuesday following complaints from neighbours, Brent council said.
The men, all of Eastern European origin, had piled bedding in every room except bathrooms, with one mattress even laid out under a canopy in the back garden.
Images from the semi-detached property show dozens of shoes piled on top of each other, clothes strewn across radiators, and junk clogging the garden.
Harbi Farah, cabinet member for housing and welfare reform at Brent council, said it highlighted the worst excesses of unscrupulous property owners.
He said: "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."
In just one room, eight men were found to be sleeping side-by-side on "wall-to-wall" mattresses.
Last week, Brent council voted in new penalty measures of up to £30,000 for people breaching housing law.
The owner of the property is being traced, Brent council said, adding it had secured more than 100 successful prosecutions against rogue landlords in July.
Mr Farah added: "Any landlord treating their tenants unfairly should be on notice, we're coming for you."