See also: Easy ways to save on your shopping
The bank account had previously offered customers three per cent interest on any savings over £20,000.
But the bank has since slashed its interest rates to 1.5%, while others have raised theirs are fall only just short of Santander.
Not only that, but Santander charges a monthly fee of £5, which means customers with low savings, specifically those with less than £5,000, could be worse off.
Martin Lewis is advising people with smaller savings pots
For example, someone with £5,000 of savings would earn £75 in interest, but would be left with just £15 once the charges had been taken off.
A comparison by Martin reveals that the Bank of Scotland Vantage easy-access account would be the best bet for people with £5,000 of savings, with its interest rate of two per cent.
A Tesco Bank account offers three per cent on up to £3,000, which would earn customers £89.
The Post Office offers 1.07%, which would earn £54, while Birmingham Midshires offers 1.3% and would earn a customer £65.
Writing on his website, MoneySavingExpert, Martin said: "For those who need easy access, are at or very near the max £20,000, earn decent cashback, and like its customer service, there's certainly no urgent imperative to ditch it.
"For those earning less cashback or with smaller savings, in most cases it really isn't worth holding on to much longer, with rates looking to go up.
"And of course if you do ditch Santander and it then decides to up its rate or drop the fee, it would almost certainly (no guarantees) let you reopen an account – though terms can change."