Spouses and civil partners are entitled to an extra Isa allowance when their partner dies. But an investigation by Which? found helpline staff at banks giving conflicting and incorrect advice.
During the mystery shop, we were given inaccurate advice in 18 out of 30 calls.
In some cases, following this advice would have meant spouses and civil partners would have had to pay tax on what should have been tax-free savings.
Here we look at how additional permitted subscriptions work, the banks that failed our mystery shop test, and how much tax you could save with the allowance.
If your spouse or civil partner died on or after 3 December 2014, you're entitled to an extra Isa allowance, known as 'additional permitted subscriptions' (APS).
How much allowance you get depends on the date of death:
For example, if your partner had £55,000 in Isa savings when they died, you'd be entitled to an extra Isa allowance of £55,000, even if you don't inherit the money.
This is in addition to your annual Isa limit (which is £20,000 for the 2019/20 tax year.)
When we spoke to one call handler at Barclays, we were told that 'there isn't any additional [allowance].'
Some other helplines also appeared to have limited or no knowledge of APS. 'This additional subscription allowance, to be honest with you, I've really never heard before,' said one call handler at Lloyds Bank.
We called the savings helplines of 10 providers - Barclays, Halifax, HSBC, Lloyds Bank, Nationwide, NS&I, Santander, Tesco Bank, TSB and Virgin Money - to test their knowledge of additional permitted subscriptions.
During some of our calls, we were transferred to a provider's bereavement team.
We told helpline staff our late uncle had £10,000 in a cash Isa with the provider, and asked whether our aunt could use her APS allowance to save money with them.
If they said she could, we asked whether our aunt's APS allowance would be affected if she only inherited half of the money from our uncle's Isa. We wanted to find out if staff could correctly confirm that it wouldn't.
We were also given conflicting advice from staff at Halifax. During one conversation we were wrongly advised our aunt's APS allowance would equal the amount she inherited from our uncle's Isa.
In fact, you don't have to inherit any of your partner's Isa savings to be eligible for the allowance.
These are just a few examples of calls where we found an inadequate knowledge on additional permitted subscriptions. None of the 10 providers we called were able to answer every question correctly in all three calls we made.
All have committed to improving their customer service after we shared our findings.
If you inherited £55,000 from your partner's Isas for example, and didn't make use of the APS allowance, you would only be able to put £20,000 into your own Isa (the annual limit).
You might then put the other £35,000 into a non-Isa savings account. If you chose the best fixed rate available at the time of writing - a five-year bond paying 2.36% - you would earn £826 interest in the first year.
The tax bill would come to £130.40 if you're a higher-rate taxpayer, or £371.70 if you're an additional-rate taxpayer.
If you're a basic-rate taxpayer you might also have to pay £165.20 in income tax if you're already receiving £1,000 interest on non-Isa savings, and you have other income that's over £17,500.
Whereas with additional permitted subscriptions, you could save tax-free.
If you want to use your APS allowance, you're free to shop around - you don't have to save with the bank your partner chose.
But under inheritance Isa rules, providers don't have to accept additional permitted subscriptions. When we surveyed 23 Isa providers we found that two - Leeds Building Society and Post Office Money - don't accept them.
Some providers also require you to open a specific inheritance Isa. You can search our table below for some of the providers that accept additional permitted subscriptions.