“The nature of NS&I’s customer base may also have shifted during 2020,” he said.
“Its savings accounts unusually found themselves near the top of the best buy tables after the pandemic hit, and banks and building societies slashed their rates.
“That kind of exposure attracts customers who shop around for the very best rates, and when things change, these savers can understandably be ruthless about moving on to the next best thing.
“It doesn’t look like it’s going to be a good year for cash savers, with markets pricing in base rate staying put, or even falling by the end of 2021, and inflation expected to rise.
“However, if NS&I continues to see such large withdrawals, it may yet raise rates before the end of the tax year. But even if it does, it probably won’t be back to the market leading levels we saw last year.”
Outside NS&I, in banks and building societies, households saved £17.6billion in cash accounts in November last year.
This was up from £12.7billion in October.