It predicted a property worth £7,500,000 now would rise to £8,424,043 in 2017 without mansion tax but just £7,601,040 if the tax was introduced.
According to the report, this could impact five year growth by an average of 5%.
The high value prime markets, that is the top five to 10% of homes by value, have already been impacted by increased stamp duty, the introduction of an annual tax on enveloped dwellings and the closure of certain tax loopholes.
Savills researcher Sophie Chick said:
"Two out of the main political parties still favour some form of mansion tax so owners and buyers will be rightly factoring it into their decisions as the election approaches.
"Rarely have the prospects for the prime property markets potentially been so dependent on the tax policy adopted by a future government, making it impossible to give a single forecast for the UK's prime housing markets without a plethora of assumptions and caveats. Much hangs on the fate of proposals for a mansion tax.
"Already we've seen previous increases in the tax burden on prime property curtail price growth in London, interrupt the flow of wealth into the prime regional and country house markets and create a two tier market above and below a £2m price threshold."
"It would take some time for the markets to accurately price in the impact of a mansion tax, but the threat of it has already slowed the market. If it becomes clear that a mansion tax is to be introduced after May 2015, we would expect an immediate price adjustment before the market more rationally finds its level,"
Chick said that increased stamp duty rates for high value homes and a clampdown on property owning non-doms had already compounded buyer caution.
Savills estimated there were about 40,000 properties valued between £2m and £3m, 30,000 between £3m and £5m. and another 17,000 between £5m and £10m.
To raise £1.2bn, the tax charges for properties worth between £3m and £5m might be in the order of £7,000 per year, rising to £125,000 for properties over £20m.
Chick added:
"Because of the expected graduated scale of charges it is likely that a mansion tax will have different impacts in different parts of the market, potentially having a more modest effect in the lower bands.
"Correspondingly, it has the potential to have more of an impact in prime London than in the lower value prime regional markets."
Lucian Cook, head of UK residential research at Savills said:
"The prime market already accounts for a disproportionately high share of the total tax take, with £2m plus sales accounting for just 0.3% of the housing market, but generating over £1bn in stamp duty."
