Savers could incur penalties when they withdraw money early from popular National Savings & Investments (NS&I) accounts.
Customers with index-linked savings who roll their money over into new certificates from 20 September stand to lose 3.2% in returns from early withdrawals.
NS&I savings certificates pay 0.25% above the retail prices index measure of inflation, which currently stands at 3.2%.
Previously, savers could cash in early after a year, and it would only cost them the extra interest on top of the index-linked amount.
This was a popular option, as savers could withdraw money from fixed accounts if they thought inflation would fall, or they needed the money early.
In future, customers will leave with just 0.19% interest on the amount they withdraw for the year. And the amount they withdraw will be stripped of 90 days' interest and index-linked returns.
If your certificate matures before 20 September however, the same rules still apply.
Savers will not be affected by changes until the end of their investment term.