London mayor Sadiq Khan said he will launch a database of criminal landlords on which they are "named and shamed" in front of the city's hard-pressed renters.
Private rents in London are significantly higher than elsewhere in the country because the city is in the grip of a severe housing shortage, with around half the number of new homes needed built each year.
The median monthly rent in London during 2016 was £1,473, according to the government's Valuation Office Agency. By comparison, the same figure for England was £650.
Some rogue landlords are exploiting the high demand by putting tenants with few or no other housing options in appalling conditions. One housing raid by council officers in Edgware found evidence of up to 40 people living in a single three-bedroom house.
"I refuse to stand by as thousands of Londoners suffer sky-high rents and horrendous living conditions in a city they call home," said Sadiq Khan, London's Labour mayor.
"Today I have seen first-hand the abysmal conditions that some of London's private renters are forced to endure as a result of rogue landlords.
"I want to be clear that the vast majority of landlords treat renters well – but a minority are exploiting their tenants and it's simply unacceptable. This must stop now."
City Hall said the new database will be built with data from London borough councils – which have a responsibility for enforcing housing law – and published on the Mayor's website. The database has the support of both landlord and tenant groups.
"The mayor's 'name and shame' online database brings information on criminal landlords and agents together to make it much easier for renters to find and avoid landlords anyone who has been prosecuted for housing related crimes," said Richard Lambert, CEO of the National Landlords Association.
"Importantly, it is also the first time renters have had a central online tool that should take some of the stress out of reporting potentially criminal housing conditions to their local authority."
"For renters to get a better deal in London, it's vital that we have a London-wide approach to dealing with rogue landlords, which is why the mayor's 'name and shame' database is so important," said Seb Klier, London Campaigns Manager at Generation Rent.
"At the moment, renters sign a tenancy with no information about whether their landlord has a good record in the market.
"This new initiative will empower renters by allowing them to check if a landlord has committed a recent housing offence, and it will put the capital's worst offenders on red alert, letting them know that if they mistreat their tenants, they will be exposed."