Brexit, which has dominated politics on both sides of the Channel since 2016, became a reality as Big Ben struck 11:00 pm (2300 GMT) in London, just as most of mainland Europe ushered in 2021, AFP reported.
Prime Minister Boris Johnson – the figurehead of the "Leave" campaign – described it as an "amazing moment" for the country and played up his upbeat narrative of a "Global Britain" unshackled from rules set in Brussels.
He vowed that post-Brexit Britain, despite being battered by a surge in coronavirus cases, would be an "open, generous, outward-looking, internationalist and free-trading" country.
"We have our freedom in our hands and it is up to us to make the most of it."
On the front page of its Friday edition, the stridently anti-EU Daily Express showed a picture of the White Cliffs of Dover and the headline "Our future. Our Britain. Our destiny".
Legally, Britain left the European Union on January 31 but has been in a standstill transition period during fractious talks to secure a free-trade agreement with Brussels, which was finally clinched on Christmas Eve.
Now the transition is over, EU rules no longer apply. The immediate consequence is an end to the free movement of more than 500 million people between Britain and the 27 EU states.
Customs border checks return for the first time in decades, and despite the free-trade deal allowing Britain continued access to Europe's nearly 450 million consumers, queues and disruption from additional paperwork are expected.
At the Eurotunnel terminal in Calais, French officials began implementing the new formalities at midnight on the dot, starting with a truck coming from Romania carrying post and parcels.
Matt Smith, managing director of HSF Logistics, which ships mainly fresh meat and chilled goods between Britain and Europe, said he was sending around 15 truckloads to the EU on New Year's Eve ahead of the changes.
The government's new post-Brexit customs systems are largely untested and Smith was doubtful how his business would fare with the new paperwork.
"We're not too sure to be honest, it seems to be a bit of a headache," he told AFP. "There'll be delays along the line at some stage."
Britain is the first member state to leave the EU, which was set up to forge unity after the horrors of World War II.
The 2016 referendum opened up abiding wounds between Leavers and Remainers, and ushered in years of political paralysis before Johnson took power last year, vowing to chart a future for Britain built on scientific innovation and new partnerships across the seas.
A parliamentary debate on Wednesday to ratify the trade deal was marked by elegiac farewells from pro-EU lawmakers, and warnings of disruption as Britain dismantles the intricate network of ties built since it joined the EU's forerunner in 1973.
Anxious wait for financial services
While the EU tariff- and quota-free trade deal averted potential business chaos in the immediate future, the divorce will play out in many practical ways.
Changes apply to everything from pet passports, to how long Britons can visit their holiday homes on the continent and an end to British involvement in a student exchange program.
Potential disruption at ports is stoking fears of food and medicine shortages, as well as delays to holidaymakers and business travelers used to seamless travel in the EU.
British fishermen are disgruntled at a compromise to allow continued access for EU boats in British waters.
The key financial services sector also faces an anxious wait to learn on what basis it can keep dealing with Europe, after being largely omitted from the trade agreement.
Northern Ireland's border with EU member state Ireland will be closely watched to ensure movement is unrestricted – key plank of a 1998 peace deal that ended 30 years of violence over British rule.
And in pro-EU Scotland, where Brexit has given a boost to calls for a new vote on independence, Johnson faces a potential constitutional headache as 2021 dawns.
Scotland's pro-EU First Minister Nicola Sturgeon promised in a tweet: "Scotland will be back soon, Europe. Keep the light on."
But opinion polls indicate that most Britons, on both sides of the referendum divide, want to move on and are far more worried about the worsening coronavirus pandemic, which has claimed 73,500 lives.