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HMRC internal manual

Inheritance Tax Manual

Does the variation affect another estate?

This applies most commonly when spouses or civil partners (IHTM11032) die within two of years of each other and the beneficiaries on the death of the second spouse or civil partner seek to vary the disposition on the death of the first spouse or civil partner to die. (IHTM35042

Although in these circumstances the effect of the variation is (usually) to reduce the entitlement of the second spouse or civil partner, a variation may also be used to increase the entitlement of someone who has died after the person whose estate is being varied.  You should make a full note of the effect of the instrument of variation (IoV) at both files setting out clearly the IHT implications.  You should use the boxes on form IOV1 and the reverse of that form for this purpose.