Extra charges will apply for anyone shopping from UK-based websites from next January.
The Government's Brexit preparations plan warns import charges, duty and VAT costs will apply similar to when ordering from the US or China.
It means people will have to pay more for any goods ordered from the UK when they arrive.
The Government plan is warning all businesses trading to, from or through the UK to prepare and has announced some financial supports for dealing with extra customs costs.
The document says: "Consumers should be aware that from 1 January 2021 purchases from or through websites in the UK will incur additional charges, duty and VAT, just as for any other third country.
"Administrative costs of export and re-import declarations in relation to returns for retailers will arise."
While it says any plant or animal products will have to undergo checks "and fees may be payable".
"In certain circumstances, EU consumer protection legislation may no longer apply to or in respect of the UK".