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HMRC internal manual

Inheritance Tax Manual

Basic principles: divorce or dissolution of a civil partnership

Where a marriage or civil partnership ends because of divorce or dissolution of the civil partnership and one of the parties to the relationship then dies, any unused nil rate band on their death is not available for transfer on the death of their former spouse or civil partner.  Unless the divorcee or former civil partner remarries or enters a new civil partnership, any unused nil rate band on their death will remain unused.

Unused nil rate band can only be transferred from one spouse or civil partner to the survivor when the relationship is brought to an end by the death of one party to the relationship.