The tax is likely to apply to the gilt option element of guaranteed bonds, so-called because they promise high levels of growth or income to investors, net of tax at the basic rate.
Kenneth Clarke, the Chancellor, may reintroduce tax on gilt options - scrapped by his predecessor Nigel Lawson - in the November Budget.
Tens of thousands of savers who have put money into these bonds would either have to pay the tax on their gains or hope the insurance companies that market them will meet the bill.
Most insurers have clauses in their contracts with savers saying their guarantees are subject to current tax laws - "which may change".
However, Scottish Widows, which has attracted more than £300m into its bond since January, said yesterday that it intended to honour its guarantee.
The Personal Investment Authority, the personal finance watchdog, is preparing to tell its members that a guarantee must be met unless the impact of any small print is made explicit.
The mooted change in tax law is among the reasons given by one insurer, Eagle Star, for deciding against launching another issue of its Guaranteed Bond.
In a letter to one independent financial adviser, Roddy Kohn, of Kohn Cougar, Eagle Star said: "Our understanding of the future intentions of the Inland Revenue . . . has matured to the point where we believe that taxation will be applied.
"The effect of this may well be retrospective even if the legislation is not, as it would take effect when the gain is realised at maturity, rather than at the inception of the investment." Other reasons given for pulling the launch include the PIA's own investigation. Mr Kohn said: "Investors should not have to shoulder the worry of this potential nightmare. Companies must step in."
Gilts, and options in them, are not at present subject to tax, usually levied at an investor's marginal rate. They became a feature of guaranteed bonds after a move to plug another tax loophole in the last Budget.
A Revenue spokeswoman said: "We are aware that many companies are using gilt options in guaranteed bonds but we are unable to say what the Chancellor may or may not do in the Budget."