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Please refer to the guidance here
If you import goods that are not controlled into Great Britain from the EU between 1 January and 30 June 2021 and delay your customs declaration you must account for import VAT on the return which includes the date you imported the goods. To complete the boxes on your return, you’ll need to estimate the import VAT due from your records of imported goods.
When you submit your delayed declaration you must select that you’re accounting for your VAT on your return. Your next online monthly statement will show the amount of import VAT due on that declaration. You’ll then be able to:
• adjust your estimate
• account for any difference on your next return
Essentially, the import VAT needs to be estimated and accounted for on the VAT return relating to the date of the import. It should not be left until the supplementary declaration is made.