Britain on Friday ends almost half a century of integration with its closest neighbours and leaves the European Union, starting a new — but still uncertain — chapter in its long history.

At the clocks strike 11pm (UK time) — midnight in Brussels — Britain becomes the first country to leave the 28-member bloc and does it alone for the first time since 1973.

Prime Minister Boris Johnson has backed Brexit since the 2016 referendum vote to leave that triggered bitterness and division but he has promised to lead the country to a bright new future.

Official celebrations will be muted out of respect for half the population who wanted to stay in the EU and who remain fearful of what lies ahead.

"Our job as the government, my job, is to bring this country together and take us forward," Johnson said in a statement to mark the historic occasion.

He added: "This is not an end but a beginning. This is the moment when the dawn breaks and the curtain goes up on a new act."

Nothing will immediately feel different thanks to a transition period negotiated as part of an EU-UK exit deal, which was ratified only this week.

Read: What is Brexit all about?

Britons will be able to work in and trade freely with EU nations until December 31, and vice versa, although the UK will no longer be represented in the bloc's institutions.

But legally, Britain is out — with no easy way back.

And while the exit terms have been agreed, Britain must still strike a deal on future relations with the EU, its largest trading partner.

Divided country

Getting this far has been a traumatic process.

Despite Britain's resistance to many EU projects over the years — it refused to join the single currency or the Schengen free travel area — the 2016 vote to leave was a huge shock.

It unleashed political chaos in London, sparking years of bitter arguments that paralysed parliament and forced the resignations of two prime ministers.

Johnson brought an end to the turmoil with his decisive election victory last month, which gave him the parliamentary majority to ratify the Brexit deal.

But Britons remain as divided as they were nearly four years ago, when 52 percent voted to leave but 48 percent voted to remain in the EU.

Johnson himself is a polarising figure but he has emphasised unity and is avoiding any major celebrations that might exacerbate divisions.

He will first host a special cabinet meeting in the northeastern English city of Sunderland, which was the first to declare for Brexit in the 2016 vote.

At 10pm (UK time), he will broadcast an address to the nation, and then host a reception for staff at his 10 Downing Street office.

A light show will also be held outside No 10, while millions of commemorative 50 pence coins are being minted to mark the occasion.

On them is the message: "Peace, prosperity and friendship with all nations."

By contrast Nigel Farage, the Brexit Party leader who has spent decades campaigning to leave the EU, is planning a large rally in nearby Parliament Square.

He gave a triumphant speech on Wednesday, his last day as a member of the European Parliament, waving Union Jack flags and declaring that Britain was "never coming back".

But afterwards, many MEPs bade Britain an emotional goodbye and broke into a chorus of the traditional Scottish song of farewell, "Auld Lang Syne".

Brussels' Grand-Place was on Thursday lit in red, white and blue.

Trade talks loom

British eurosceptics have campaigned for decades to free their country from what they see as an overly bureaucratic and unaccountable institution.

Johnson has promised to leave the EU's single market and customs union, giving Britain the freedom to forge trade deals around the world.

But exactly how that will work — and at what cost to firms that depend on EU business — remains to be seen.

The EU divorce deal resolved the issue of London's debts, the rights of EU expatriates, the status of Northern Ireland and the transition period.

But Johnson has given himself just 11 months — to the end of the transition — to negotiate a new EU partnership, covering everything from trade to security cooperation.

He will set out his plans in a speech early next week but Brussels has already warned he must either limit his ambitions or ask for more time.

Johnson also wants a trade deal with the United States, where President Donald Trump is a huge supporter of Brexit.

Secretary of State Mike Pompeo hailed the potential opportunities during a visit to London on Thursday, saying: "You will see the enormous benefits that accrued to both of our nations as a result of this."