London, United Kingdom – The UK is bracing for a historic break from the European Union that will reset the relationship between them for generations to come.

More than four years after a slim majority of Britons voted in favour of quitting the EU, the UK will leave the bloc’s single market and customs union at 23:00 GMT on Thursday, December 31.

As the new year starts, the two sides must abide by the terms of a recently inked deal, which sets boundaries on their trade and security relationships.

On Wednesday, the UK Parliament speedily approved the deal, meaning it has now passed into UK law.

In short, Brexit marks the most momentous shift in the country’s recent history and will reverse decades of closer economic, cultural and social integration with the EU, its largest trading partner.

“Historically it’s really significant – it is the first major example of building barriers to trade and to cooperation that we have seen in the modern period,” Simon Usherwood, a professor of politics at the University of Surrey, told Al Jazeera.

“And there’s still a kind of uncertainty regarding whether this is about Britain retreating from the world and pulling up the drawbridge or becoming an international player in the broader sense and becoming a ‘global Britain’.”

The 1,240-page Trade and Cooperation Agreement was finally brokered a week ago, following months of fractious negotiations in the so-called transition period, which began after the UK’s formal departure from the EU in January.

After the deal was reached, a sense of relief was felt by both sides.

It averts the prospect of a chaotic divorce and ensures goods can continue to travel between the UK and the EU without tariffs or quotas from the beginning of 2021, smoothing trade worth hundreds of billions of pounds – and euros – a year.

Still, London’s imminent departure from Brussels’ orbit will bring about a raft of new rules and red tape for businesses.

How Britons and Europeans live, work and travel between the country and the continent will also change, with new visa regulations.

‘An economic rupture’

In the wake of the deal being reached, UK Prime Minister Boris Johnson spoke of a “newly, and truly, independent nation”.

He won by a landslide in the December 2019 election on a pledge to “Get Brexit Done”, after the issue effectively ended the political careers of his two predecessors – Theresa May and David Cameron.

Johnson has hailed the EU deal as a triumph, claiming he has achieved what he set out to achieve.

The UK, he said, has taken back control of its laws, borders, and fishing waters under the pact.

He also stressed that the UK will be the “best friend and ally the EU could have” as the pair reorient themselves to their new relationship.

But opponents of Brexit say the divorce threatens to break up the UK, harm the economy in the long term, and diminish its global standing.

National economic output will shrink by 4 percent over the next 15 years as a result of the UK’s departure from the EU’s single market and customs union, according to the UK’s financial watchdog, the Office for Budget Responsibility (OBR).

EU economies are also predicted to take a hit, although the impact of Brexit will vary considerably across the bloc.

Ireland, Belgium and the Netherlands are expected to be most affected, as they do the most trade with the UK.

Analysts said that even with the agreed trade deal, Brexit would effectively result in a lose-lose financial outcome for both sides.

“It’s an economic rupture … and the biggest one-day change in trading relations in modern history,” David Henig, director of the UK Trade Policy Project at the European Centre for International Political Economy think-tank, told Al Jazeera.

“There’s 660 billion pounds [approximately $898bn] worth of trade which tomorrow is under new, more restrictive rules,” he said. “Just how much impact it will have we simply don’t know – it’s an entry into the unknown.”

Henig and Usherwood also expect continued political fallout next year and beyond, citing possible flash-points: disruption accompanying the implementation of the trade deal, the agreement’s protocol for Northern Ireland, and its implications for Scotland where the ruling nationalist party is pushing for a second referendum on independence.

There will also be further wrangling between London and Brussels, with many aspects of the pair’s overall future relationship still left to be worked out.

“If you assume the end of Brexit is when there is a new, stable relationship with the EU then we are not yet at that point,” Usherwood said.