Thousands of bereaved savers are missing out on a tax break by not taking advantage of rule changes introduced in the Budget in 2015.
Under the additional permitted subscription rules, the partner of a deceased person can transfer the deceased's Isa account to their own name without incurring a tax charge.
The rule applies to spouses and civil partners and allow for the creation of a one-off Isa allowance to be received by the partner, which equals the amount held in the Isa account of the deceased.
But data obtained from HM Revenue & Customs by Zurich under freedom of information rules show the entitlement has been claimed by just 21,000 people in the 2017 to 2018 tax year.
This was less than the 25,000 who claimed the tax break in the previous year.
There are 21 million Isa accounts in the UK.
Alistair Wilson, head of retail platform strategy at Zurich, said: "Despite being in its fourth year, the take-up of this tax break looks shockingly low. People who miss out on the allowance will be hit by a tax bill that quickly eats into the returns on their savings and slows down the growth of their nest egg.
"Although it is not nice to think about a loved one passing away, speaking to a financial adviser can help you pass on wealth efficiently and ensure you or your family don't pay any more tax than needed."
Under the rules, if your partner had £50,000 in Isa savings when they died, you would be able to invest £50,000 tax-free on top of your own £20,000 annual Isa allowance – taking your total allowance for the year to £70,000.
Danny Cox, head of communications at Hargreaves Lansdown, said: "Preserving the tax benefits of an Isa after death is pretty straightforward, but does require intervention and action. Isa providers and executors have to play their part to ensure survivors are aware of this valuable tax break.
"Couples typically manage their Isas as one portfolio, so the ability to pass on an Isa tax shelter removed a quirk of the tax system which was the source of deep frustration and additional cost for surviving spouses."