Rogue landlords were issued fines totalling more than £250,000 by Barking and Dagenham Council last year, it reports.
The local authority also carried out 14 operations with the Metropolitan Police, including one at a suspected brothel operating as an illegal house in multiple occupation (HMO) in Reede Road, Dagenham.
Other offences uncovered around the borough included not having a landlord licence and ignoring enforcement notices.
Cllr Margaret Mullane, cabinet member for enforcement and community safety, said: "The majority of landlords understand their responsibilities – both legally and morally – to their tenants and provide much needed homes that are safe to live in.
"However, there are some unsavoury landlords who choose to flout the rules and that is where we will use all the legal powers available to us to take action."
The council hails itself as the first in the country to make it mandatory for all private landlords in the borough to have a licence.
There have been about 15,000 applications for licences since the scheme launched in 2019, it reports.
According to the local authority, it is estimated that about a quarter of all housing in Barking and Dagenham is in the private rental sector.
But the council says some landlords still ignore the rules, pointing to one unlicensed property in Mayfair Avenue, Chadwell Heath, where inspectors found 13 people in a four-bedroom house.
Landlords whose properties fail to meet a required standard can be issued an enforcement notice, giving them a set period to carry out remedial work.
If they ignore this notice, which carries a fine of £520, they can be issued a civil penalty notice (CPN) of up to £30,000.
Last year, the council issued 117 CPNs amounting to £271,310 and 89 enforcement notices totalling more than £46,000.
The council investigated 99 cases of illegal eviction and handled 122 cases of landlord harassment in the past year.
Cllr Mullane added: "Our pioneering licensing scheme has helped drive up the standards of private rental properties in our borough, but as some of these hefty fines show, we won’t hesitate to take action against the few who continue to put profit ahead of people and leave residents living in dangerous, poorly maintained homes."
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