That’s three years quicker than they would under the current £15,240 ISA allowance, according to rplan.co.uk, the online investment platform.

It found that people who drip feed the new ISA allowance, for example invest £1,666 a month, into a medium risk portfolio, could make half a million pounds and become so-called ‘demi ISA millionaires’ six years quicker than those who invest their money in the current best buy easy-access cash ISA, which pays 1.4%.

Those who are more risk averse could still become demi-ISA millionaires in 17 years using a lower risk portfolio, the analysis suggests.

The ISA allowance increases to £20,000 from April 2017.

Nick Curry, director at rplan.co.uk, said: “ISAs provide great flexibility for savers, especially those who have their eyes set on becoming a demi-ISA millionaire. 15 years is not that long over the course of a lifetime; to give yourself a good chance, start as early as possible, take advantage of as much of the ISA allowance as you can, and consider investments over savings accounts if you are comfortable taking a bit more risk. Once you are half way there, becoming an ISA millionaire doesn’t seem too far off.”