FCA officials said the sector will begin adapting ahead of the rules, as it did with similar rules in the European Union.
They rejected concern among lawmakers the new regime would create "bubbles" as money flocked to the fewer funds that qualify as sustainable.
Lawmakers on the committee examining the FCA's proposals put Cummings on the defensive over his criticism that too many funds will be excluded.
Kate Levick, associate director of sustainable finance at think tank E3G, told the hearing the FCA's plan "would remove the significant amount of greenwashing currently in the market".
($1 = 0.8290 pounds)Reporting by Tommy Reggiori Wilkes and Huw Jones; Editing by Mark Potter and Emelia Sithole-MatariseOur Standards: The Thomson Reuters Trust Principles.