Boris Johnson has said the UK has “freedom in our hands” and must make the most of it as the country prepares to leave EU trading rules at 23:00 GMT.
Historic changes to rules on travel, trade, immigration and security are to come into force as the UK enters a new era of relations with the continent.
UK officials have insisted new border systems are “ready to go” amid concerns about possible delays at ports.
The PM said the UK could now do “things differently and if necessary better”.
The trade deal agreed by the two sides on Christmas Eve, and passed by MPs on Wednesday, avoids the need for import taxes – tariffs – after the UK leaves the EU’s internal market and customs union on 1 January.
While fears of giant tailbacks of lorries at Dover have receded, uncertainty remains about new customs rules and the government has warned there will be “some disruption” in the coming days and weeks.
To reduce the risk of delays, the UK is phasing in checks on goods entering the country from the continent over a six-month period up to July 2021.
But some new customs procedures will come into force on the UK-side immediately from 11pm, such as on imports of alcohol, tobacco, chemicals and controlled drugs.
EU member states are introducing full customs declarations and other controls on UK exports from 23:00.
Among the other things that will change from 23:00 GMT:
Intensive preparations have been taking place over the past two weeks to prepare the UK for the coming changes.
The UK’s Countdown Plan has involved operational testing of infrastructure at the border and close co-operation with France, Holland and Belgium.
A government spokesman said: “The border systems and infrastructure we need are in place, and we are ready for the UK’s new start.”