Around 150,000 married ISA savers die each year, according to Government statistics, although current rules mean their savings lose the tax-efficient wrapper upon death.
However, from 6 April, the surviving spouse will be able to increase their ISA savings by the value of the deceased partner's ISA balances.
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The option is available for ISA holders who have passed away on or after 3 December 2014, if they were in a marriage, or civil partnership, and not separated.
The region with the highest average cash ISA balance held with Nationwide Building Society on death last year was London at £16,674. Second was Northern Ireland with £16,521.
The area with the lowest recorded ISA balances upon death last year was the North West, where the average ISA balance upon death was £13,540.
Nationwide said its customer data showed the average customer with a cash ISA on death during 2014 was aged 81.
The list of regions and average ISA balances is:
Greater London: £16,674
Northern Ireland: £16,521
East Midlands: £14,515
Scotland: £14,362
South East: £14,981
South West: £14,918
North East: £13,970
West Midlands: £13,964
Wales: £13,943
North West: £13,540
Nationwide, which campaigned for ISA parity, said it accounted for more than 20 per cent of market change in ISA balances at the end of last year (November), opened 550,000 new ISA accounts between April and September 2014.
Richard Napier, Nationwide's director of savings, said: "Changes to the ISA rules are entirely welcome as people will be able to take comfort that their surviving partner will be able to reinvest the full value of their ISA savings in a tax-efficient way.
"This is incredibly important, particularly as the majority of customers will be well into their retirement – a time when finances might be tighter."