Homes in London and the South East would account for 96% of the 108,000 households liable to pay a levy on property worth over £2m.
Those over the threshold would have to pay £15,000 annually.
Lawrence Hall of Zoopla said: “The introduction of a mansion tax would disproportionately penalise homeowners in London and the South East who are already responsible for the vast majority of property tax take in the UK.
“With more than 100,000 homes to be affected by this new levy, it is somewhat misleading to call it a ‘mansion tax’ when many three bed family homes in London and the South East would find themselves caught by it.”
If Labour’s proposal goes ahead they could rein in £1.6bn in extra tax revenue per year.
Peter Rollings, chief executive of Marsh and Parsons, also attacked Labour’s plan.
He said: “Labour’s mansion tax proposals would not only injure London’s international reputation and prestige as a city open for business and investment, but would lumber the capital’s homeowners with an even weightier tax burden and potentially stifle the market.
“Any policy initiatives should concentrate on nurturing the embryonic buds of growth outside of London rather than drastically pruning back healthier branches of the market.”