While just 247,000 people paid CGT in the 2007-08 tax year, the number could be in the millions if the tax-free allowance is reduced, according to the fund management group Fidelity.
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Although the coalition Government has remained silent on the annual tax-free allowance so far, the Liberal Democrats have previously proposed reducing it to just £2,000 from the current level of £10,100.
Typical investors could also face their first tax bill much sooner if the allowance is cut.
Currently, the average saver would not currently have to pay CGT if they sold or switched their investments within the first nine years, Fidelity calculated, as it would take this long for investment gains to exceed the tax-free allowance. However, if the CGT allowance were reduced to £2,000, CGT would become due after just two years.
These figures assume annual investment growth of 5pc and are based on the company's findings that its clients had on average managed to save a pot of around £18,000 in investments outside tax-free wrappers such as Isas.
"Savers who have never previously had to complete a tax return may now have to do so, with the amount they owe dependent on how much the government raises CGT in the next Budget," Fidelity said. The Coalition is widely expected to raise CGT to a top rate of 40pc or 50pc.
Paul Kennedy, the head of tax planning at Fidelity, said: "The forthcoming increases in the rate of CGT have been subject to much attention in relation to wealthy individuals. However, it should be remembered that the allowance is relevant to all investors, not just those on higher incomes.
"Our analysis shows that decreasing the tax-free allowance will result in millions of average long-term savers being dragged into a net that many believed was being set only for the rich. Such a change is likely to damage particularly older people who have saved prudently to supplement their income in retirement and gradually sell down their holdings.
He added: "The CGT allowance must be maintained at a level where it neither produces disproportionate burden on the modest investor nor distorts or compromises sensible long-term investment plans."