What will happen on 1 January 2021?
The transition period stipulated in the Withdrawal Agreement ends on 31 December 2020. It is no longer possible to extend this period. This would have required a joint decision on an extension to have been taken by 1 July 2020. The UK let this deadline pass. As of 1 January 2021, the UK will thus no longer be part of the single market or the customs union.