Brexit supporters wave balloons and flags from Westminster Bridge last June. PA Archive / PA Images PA Archive / PA Images / PA Images

Updated at 7.10am 

TODAY AT AROUND 12.30pm, British Prime Minister Theresa May will trigger Article 50.

That will formally begin the two-year negotiations between the United Kingdom and the European Union, where regulations around trade, fishing boundaries, visas and endless other regulations will be renegotiated.

In the lead up and aftermath of the triggering of Article 50, the world will be watching. An indication of how the crucial Brexit negotiations will go will be measured in how Europe’s leaders react (with congratulations, or cynicism?) and how the markets behave.

US President Donald Trump’s commentary will also be influential, as always.

So what is Article 50?

Article 50 is a part of the Lisbon Treaty, or the European Union agreement, that allows a country to leave the EU.

It was added after Greenland had to get the approval of all other EU member states before they left in 1985.

Article 50 is as follows:

1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.

The UK is bound by their constitution to carry out the results of referenda (Ireland is not similarly bound by their constitution).

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2. A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union… It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.

According to the Guardian, the UK’s representative in Brussels Tim Barlow is to personally hand over a letter from the British government to the European Council president, Donald Tusk.

3. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.

This is significant – although there is much reportage about the two-year period assigned to renegotiate a new relationship between the UK and the EU, in reality the time can be extended.

But the longer negotiations take, the worse off both sides will be, as it creates uncertainty for businesses looking to invest or open operations in the EU or the UK.

No turning back now

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Once it’s activated, there’s no turning back, say UK and European lawyers, which is why activating Article 50 is considered to be so significant.

If Britain decide they want to rejoin the EU after triggering Article 50, it means they will have to reapply like any other country, as stated below:

If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49.

(Article 49 says: “Any European State which respects the values referred to in Article 2 and is committed to promoting them may apply to become a member of the Union.” )

Britain will be excluded from EU talks once Article 50 is triggered. Stefan Rousseau Stefan Rousseau

So triggering Article 50 is quite a straightforward process, considering it hasn’t been done before.

What’s less clear is how negotiations will go, and what the reaction will be to the official beginning of Brexit.

Reaction

After Article 50 is triggered this afternoon, the reaction of EU leaders to the action will be a good indicator of how negotiations will progress.

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Donald Trump’s comments will also be significant (he’ll probably congratulate Theresa May and make a jab at the EU).

Scottish First Minister Nicola Sturgeon’s reaction will also be significant – its parliament last night voted to make a formal request to the British government to hold a new referendum on independence.

Here at home, Taoiseach Enda Kenny and relevant ministers are expected to issue their responses this afternoon.

The markets are also expected to react: billions were wiped off the British stock exchange in the immediate aftermath of Brexit, followed by a dramatic surge two days later.

“First the panic effect, then the rebound. That’s a well-known mechanism on financial markets,” said Christopher Dembik, an economist at Saxo Banque in Paris said at the time.

Read: May and Sturgeon discuss future of UK – ‘moronic’ Daily Mail headline focuses on their legs

Read: Theresa May is heading north for a Scottish independence showdown with Nicola Sturgeon