The average interest rate for a cash ISA, or Individual Savings Account, dropped to 1.43 per cent in August.
It's the smallest reading since the Bank started calculated this average in 2011.
Returns from notice accounts, including ISAs, are at their lowest since records began in 1999.
Interest rates have been at rockbottom levels for 6 years, but banks and building societies have trimmed them even further in recent months.
Two weeks ago 400,000 savers with National Savings & Investments ISAs were told their rates would be cut from November.
Mortgage borrowers are benefiting from the same trend, with their interest costs also at the lowest level in recent records.