More than 400,000 landlords will be pushed up a tax bracket from April next year.
By 2021, landlords will not be able to deduct mortgage interest payments or other financial costs from their turnover before declaring their taxable income.
The changes are being phased in, and this means 440,000 landlords who are paying the basic rate of tax at the moment will end up paying a higher rate by next year, according to the National Landlord Association (NLA).
Landlords' tax liabilities will increase most in central London (up by 31 per cent) and the East of England (up by 30 per cent).
Richard Lambert, chief executive of the NLA, said: "When the government announced these changes last year, it claimed they would only hit a small proportion of higher-rate tax payers.
"We now know that is complete tosh.
"The government must look to amend these tax changes and minimise the impact on landlords and their tenants.
“Unless this happens, landlords will face an impossible decision of whether to increase rents and cause misery for their tenants, or to sell-up, and force their tenants to find a new home”.