According to a committee of MPs, loyal bank customers receive "measly" rates of interest on their savings.
According to members of the Treasury Committee, the trend, which disproportionately affects older and more vulnerable customers, is the result of banks seeking higher profit margins.
Instant access savings products offer rates that are significantly less than the Bank of England base rate.
Banks claimed they had to strike a balance between borrowers' and savers' needs.
The returns offered to their savings customers have been a point of contention for the committee's MPs over the past few months.
The heads of the UK's four largest banks—Lloyds, NatWest, HSBC, and Barclays—were interrogated in Parliament in February.
The four bank bosses, who were described as the highest-paid panel that had appeared before the committee in a while and who earned more than £10 million annually collectively, claimed that the debate had been mistakenly centered on the interest rate offered on easy access savings accounts.
They argued that instant access products frequently served as a "gateway" to deals with higher interest rates, while regular saver deals provided market-leading rates of interest.
A series of letters were sent to those banks and the providers in the next tier after the hearing. Just recently, their responses were released.
One of them was a letter from Debbie Crosbie, chief executive of Nationwide, in which she emphasized the need for balance, caution, and the building society's efforts to help customers find better deals.
Taking into account "the interests of savers with our mortgage borrowers, particularly with cost-of-living pressures," was one of the factors she mentioned.
The providers' defense does not seem to have impressed the committee. In contrast to the Bank of England base rate of 4.5 percent, the big four High Street banks offered instant access savings rates ranging from 0.7 percent to 13.5 percent.
The committee's chair, Harriett Baldwin, declared that it is more obvious than ever that the biggest banks in the country need to step up their efforts to promote saving.
The meager easy access rates offered make us believe that devoted customers are being penalized in order to increase bank profit margins, even though other products are accessible to those who shop around.
"We continue to be concerned that the loyalty penalty is particularly pronounced for elderly and vulnerable customers who may still rely on High Street bank branches," the statement reads. ".
The trade association for the banking industry, UK Finance, released data on Wednesday that revealed total household savings decreased year over year for the first time in at least 15 years.
As people dipped into their savings accounts to pay for larger bills and food purchases, the value of deposits in instant access accounts decreased by 4 percent in March to £867 billion from £905 billion a year earlier.
