The limit under the Financial Services Compensation Scheme - the value of accounts with a bank or building society that is protected if the bank gets into trouble - will tumble from £85,000 to £75,000 from the start of next year.
But savers hit by the fall in the compensation limit can move up to £10,000 from their account without charge from October, the Bank of England has said.
The surprise cut will expose savers who have put more than the new limit into fixed-rate bonds that don't reach the end of their term until after this date.
You can only move as much as necessary to bring you back down to the new £75,000 compensation limit. And you must do it all in one go to ensure you don't face a penalty. Thus, if you have £80,000 with one bank or building society, the maximum you can move is £5,000, but if you have £85,000 you can move the full £10,000.
Under Bank of England rules, banks and building societies can't force you to move all rather than just part of your money in a fixed-rate bond and suffer a rate cut on your whole balance.
This applies even though terms and conditions on your account deem you can't make partial withdrawals during the term.
However, you could lose out on the interest you expected when you move part of your money, because rates on offer now are lower than on your original bond.
For example, three years ago you could pick up a five-year fixed-rate bond at 4 per cent (3.2 per cent). But the best two-year deal for a switch now is 2.38 per cent (1.9 per cent). By switching £10,000, you will lose a total of £324 before-tax interest.
The window to move runs until 31 December. Once you have put in a request, banks have up to two months to return your money - or by the end of January next year, whichever is sooner.