London has lost 30,000 millionaires over the past decade, including more than 11,000 in the last year alone, according to analysis for the Evening Standard.
The research, conducted by New World Wealth for Henley & Partners, suggests the outflow could be as financially damaging as the departure of 1.47 million average taxpayers.
Each wealthy individual is estimated to have contributed nearly £394,000 annually in income tax, based on calculations by the Adam Smith Institute.
The capital's shrinking millionaire population has been linked to rising taxes under successive governments, Brexit-related economic instability and the weakening pound.
Recent moves by Chancellor Rachel Reeves to tighten rules on non-domiciled residents have also prompted high-net-worth individuals to relocate abroad.