As at 31 August, the average instant access account available from banks and building societies paid 0.22%.

But data from 30 September revealed that the average instant access account paid 0.27% – just 0.05% more.

This is despite the Base Rate having risen 0.25% to 0.75% in August.

James Newbery, investment manager at property investment platform British Pearl, said this is another example of the banks looking after themselves with little concern for hard-hit savers.

“There may be business decisions why the whole of August’s interest rate rise can’t be passed on, but for just a fifth of it to find its way to depositors is nothing short of insulting.

“It’s just more evidence that those seeking returns that can shelter their capital from inflation need to do a lot more than just walk down the high street or select traditional accounts that haven’t served their customers well for over a decade.

“Savvy savers must continue to look for avenues they may not have considered before or risk seeing their nest eggs slowly evaporate before their eyes.”

However, just last week US banking giant Goldman Sachs launched a market-leading easy account paying 1.5%.