As the clocks struck midnight to herald New Year's Day, 1973, a Union flag was raised over the headquarters of the European Economic Community to mark the moment Britain officially joined the European project.

George Thomson, one of the UK's first European Commissioners, took part in a torch-lit procession through Brussels to celebrate what Britons hoped would be the dawn of an exciting and prosperous new age.

On Wednesday the mood in the Belgian capital will be altogether more sombre as Sir Tim Barrow walks those same streets carrying the document that will officially confirm Britain is ending its 44-year relationship with Europe.

Sir Tim, Britain's most senior diplomat in Brussels, will hand-deliver Theresa May's historic Article 50 letter to the European Council president Donald Tusk during a day of highly stage-managed political theatre.

Advertisement

The era-defining document will already be in Belgium on Wednesday morning, and its all-important contents - May's opening gambit for Brexit negotiations - will finally be released to the public within minutes of Tusk receiving it at lunchtime.

That will not only start the clock on two years of talks over Brexit, but will also give MPs their first chance to interrogate May over her Brexit policy in what is likely to be a marathon session in the Commons.

European Council President Donald Tusk is expected to tweet when he receives the letter from Theresa May. Photo / AP
European Council President Donald Tusk is expected to tweet when he receives the letter from Theresa May. Photo / AP

The Prime Minister signed the most important document of her career in Downing Street on Tuesday afternoon.

After she posed for a picture signing it, the letter, which runs to several pages, was taken to Belgium overnight by a civil servant accompanied by a guard, to ensure there could be no last-minute hitches. It is now in the possession of Sir Tim, the UK's permanent representative to the EU.

While the letter was in transit on Tuesday night, May made "good will" phone calls to key European leaders including Tusk; Angela Merkel, the German Chancellor; Francois Hollande, the French president, and Jean-Claude Junker, the European Commission president.

Tusk confirmed via Twitter just after 6.30pm last night that his own phone call had already happened. On Wednesday morning May will hold a special meeting of the Cabinet at 8am, in the same way that she does on budget day, to give ministers a final briefing on the contents of the letter and on what happens next.

As she begins her weekly Prime Minister's Questions in Parliament at noon, Sir Tim will set off on his short journey in Brussels. Details of exactly how and where he will meet Tusk, as well as his planned route, have been kept tightly under wraps because of fears that Remainers might try to intercept the letter in a final act of defiance.

However if Sir Tim follows normal protocol for the delivery of important documents - though there will be nothing normal about his job on Wednesday - he will leave his office in central Brussels and walk the 150 yards to the European Council building where Tusk has his own office.

Advertisement

The handover is due to take place at 12.30, at which point Tusk is planning to Tweet the fact that he has received it. The two-year countdown to Britain's exit from the EU will officially start the moment Tusk has the letter in his hand.

He will pose for posterity receiving the letter from Sir Tim, then walk to the front of the Europa building - also known as "the egg" - part of the EC complex, to make a brief statement. At the same time, May will make a statement to the House of Commons confirming that the Brexit process has begun and setting out the contents of the letter.

Downing Street will publish the letter and place it in the Commons and the Lords once the Prime Minister has spoken.

Sir Tim Barrow the UK's permanent representative in Brussels. Photo / AP
Sir Tim Barrow the UK's permanent representative in Brussels. Photo / AP

Although no details of the letter have been released in advance, insiders have suggested it will broadly stick to the principles set out by the Prime Minister in her landmark Brexit speech at Lancaster House, London, in January.

The key points she raised during that speech included controlling our own laws, strengthening the Union, maintaining the Common Travel Area with Ireland, controlling migration and co-operation in the fight against crime and terrorism.

The most contentious points will be around migration and trade: May will fight for a free trade agreement with Europe and controlled migration, but EU leaders have made it clear that access to the single market is only possible with freedom of movement for EU citizens.

In the Commons Jeremy Corbyn, the Labour leader, will respond to May's statement, beginning an afternoon of interrogation for the Prime Minister in the Commons that is scheduled to last for at least 75 minutes.

As soon as May's grilling is over, she and members of the Cabinet will begin calling their counterparts across the EU to begin the Government charm offensive.

Meanwhile, in Brussels, the leaders of political parties in the European Parliament will meet at 2pm UK time to discuss the letter and finalise their "red lines" for the negotiations to come.

Guy Verhofstadt, the European Parliament's chief Brexit negotiator, and Antonio Tajani, President of the European Parliament, will address the media at 3.30pm, giving the first official reaction to the contents of the letter.

On Wednesday evening Tusk will fly to Malta for a Congress of the European People's Party, the coalition of centre-right political parties, at which he will have a chance to discuss the Article 50 letter with delegates including Merkel, Junker and the Irish Taoiseach Enda Kenny. Tusk will send his formal reply to May on Friday.

What is Article 50?

Article 50 of the Treaty of Lisbon gives any EU member the right to quit unilaterally, and outlines the procedure for doing so

There was no way to legally leave the EU before the Treaty was signed in 2007

It gives the leaving country two years to negotiate an exit deal

Once set in motion, it cannot be stopped except by unanimous consent of all member states

Any deal must be approved by a "qualified majority" of EU member states and can be vetoed by the European Parliament

- Originally published in Telegraph UK