Foxtons has lost an appeal against the £35,000 fine it was hit with at the end of last year.
The agency was fined in December for failing to provide Tower Hamlets Council with information which it requested about a property's safety.
Over an 18-month period, Foxtons failed to provide the authority with safety documents for a property and therefore could not prove it was safe for tenants to live in.
Foxtons appealed the fine, which was rejected and the council has subsequently been awarded £820 in costs.
"We believe in supporting private renters so it was important to pursue this case and send a strong message to landlords and agents that we will protect renters in the borough," said Councillor Sirajul Islam, cabinet minister for housing in Tower Hamlets.
"I’m pleased that the appeal has upheld the council’s position.”