The trend emerged as some of the region's biggest lenders outlined a profitable start to the year in results that also offered a glimpse of a phenomenon dubbed a "bank walk" - a slow but notable outflow of customer cash.

Lenders wasted little time in charging more for loans when interest rates rapidly rose from an almost 15-year slumber around zero last year, but most have dragged their feet on boosting deposit rates paid to millions of their customers.

That has boosted profits at many major banks beyond many analysts' expectations but left savers disgruntled, raising fresh questions over the longer-term stability of the sector.

"Traditional banks need to decide whether to maximise their return by keeping rates on deposits as low as possible, or to prioritise their liquidity and stability by increasing rates and retaining customers' funds," Nicola Marinelli, assistant professor of finance at Regent's University London, said.