Figures from the Bank of England showed that households added £8.1bn to banks and building societies in November, up from £6.7bn the previous month, with £5.1bn of that flowing into ISAs.
Ian Futcher, a financial planner at Quilter, said the surge was fuelled by speculation ahead of the late-November Budget.
“Rumours were rife in the run-up to the Budget, and talk of potential changes to ISAs under Rachel Reeves appears to have prompted more people to boost their savings,” he said.
Laith Khalaf, head of investment analysis at AJ Bell, said speculation that the cash Isa allowance would be cut ahead of the Budget was a key factor behind the surge.
“That did turn out to be true, although the lower limit won’t be introduced until 2027 and over-65s will be exempt,” he said. “Many savers clearly didn’t want to take any chances and moved quickly to use their cash Isa allowances before the Budget, in case changes were brought in straight away.”
The latest Money and Credit figures also showed net mortgage approvals slipped to 64,500, while net mortgage lending to individuals edged up only slightly to £4.5bn in November.
Consumer credit increased to £2.1bn during the month, from £1.7bn in October, with credit card borrowing the main driver, rising to £1bn from £0.7bn.