Those 53 letting agents whose websites were not compliant with legal requirements are now under investigation as their websites fail to state what fees they may charge tenants and landlords.
Brent and Harrow Trading Standards are also cracking down on letting agents that are not registered with approved money protection schemes or registered with a redress scheme.
The Trading Standards team has sent 293 advisory letters to letting agents across both boroughs warning agents of the actions they need to take.
Officers have also carried out inspections of 49 letting offices in Brent and Harrow.
Nine businesses have been issued with financial penalty notices, and six have received formal written warnings. Others are still under investigation.
The National Trading Standards Estate and Letting Agency Team (NTSELAT) is working with local authorities and the six approved CMP providers to spot those who have not signed up as well as those not displaying their chosen scheme by providing a step-by-step guide to the legalities and requirements of regulating letting agents.
Trading Standards Officers have the enforcement to impose fines of up to £5,000 for any firm that fails to display this information.
Under the Tenant Fees Act 2019, which came into force on June 1, agents are banned from charging all but a small handful of fees.