Savers are losing £3.5bn a year in interest due to switching inertia.
Many people are loyal to high street banks, but these often offer worse rates than smaller, rival firms.
Customers with Barclays, HSBC, Lloyds, Royal Bank of Scotland and Santander have £827bn of the total £1.3 trillion saved in the UK, according to the Centre for Economics and Business Research, a consultancy.
The study, commissioned by savings platform Flagstone, found that these customers would make £3.4bn in interest on their savings over the next year. But this would more than double, to £7bn, if these savers swapped to better deals with rivals.
Santander has the highest interest rate for any instant saver account from the high street banks listed, at 0.4pc. But Virgin Money pays 1.5pc for a similar deal.