On Wednesday, the UK Prime Minister is scheduled to “trigger” Article 50 of the Treaty on European Union – the TEU - to announce to the 27 other EU Member States that the UK intends to leave the EU.

It will be no surprise. 277 days ago, on 23 June last, a majority of voters in the U.K. and Gibraltar voted in favour of leaving. On 2 October last, Theresa May announced her intention to “trigger” Article 50 before the end of this month. Everyone saw the "triggering" coming!

Triggering Article 50 is big news now but it will be largely forgotten. If the UK leaves then it will be the tough negotiations and, moreover, the final deal which will be remembered - triggering Article 50 is like the kick-off of a football match, it will be the result that will be remembered. If the UK changes its mind – and States do (Norway changed its mind twice about joining the EU and Indonesia changed its mind about leaving the UN) – then triggering Article 50 will also be largely forgotten.

A Member State leaving the EU is an unprecedented move. While Algeria, Greenland and Saint Barthélémy were all in, what is now, the EU, none was a Member State or such a strategically important player as the UK. But what are the legal implications of what has happened?

At one level, nothing changes legally because of the triggering of Article 50. The UK is still bound by EU law – emphasised recently by the European Parliament – and, legally, it is “business as usual” with the UK obliged to comply with EU law while businesses and citizens in the U.K. may avail of EU rights and others elsewhere may still invoke EU law against the UK.

In reality, the shadow boxing is over. The hard negotiations starts. There will be even more uncertainty and volatility. The court challenges will continue – the May Government has lost before the UK Supreme Court already which means that challengers may take heart. The reality is biting that the phoney war has ended.

Legally, what will the UK do on Wednesday?

Under Article 50(1) of the TEU, the UK will announce its intention to withdraw from the EU.

The UK will, under Article 50(2), notify the European Council of its intention to leave. The European Council is composed of the Heads of Government of the Member States (except France which sends, as its representative, the President who is the Head of State). Legally, this notification under Article 50 will trigger the withdrawal process.

Under Article 50(3), the EU “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”. The EU will use guidelines provided by the European Council which have been drafted over the last 9 months and are expected to be issued 48 hours after the UK triggering Article 50.

The agreement would be concluded on behalf of the EU by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament. As unanimity is not required, some or all Member States may have to accept an imperfect deal. So, for example, Ireland may find that it opposes elements of the deal but find that it has been approved by a qualified majority.

Many commentators assume that the UK will leave the EU years exactly two years from the triggering of Article 50 – two years from Wednesday. It is not so simple. The EU Treaties would normally cease to apply to the UK from the date of entry into force of the withdrawal agreement concluded between the EU and the UK – which could be earlier or later than two years after the triggering notification. However, if no such agreement is reached then the EU treaties would cease to apply to the UK exactly two years after the notification to leave. But this rule is also subject to an exception. The 27-member European Council (without the UK), may in agreement with the UK, unanimously decide to extend this two year period. So, even the “two year” rule is not so certain or simple.

By triggering Article 50, the UK is left out of the negotiations on the EU side – a situation that it is getting used to over the last few months as the “27” develop their own strategy in meetings from Bratislava to Brussels. Could the UK withdraw the notification and change its mind? Views differ whether this is legally possible but, on balance, the UK could change its mind and the “law would probably follow the politics” and allow them to stay. That might happen if there could be sufficient reform in the EU to win a majority vote in the UK to remain or if the final negotiated deal was too unpalatable for the UK to accept and staying was a better option.

If the UK leaves and decides to re-join the EU then they have to apply in the normal way to the join the EU according to Article 50(5).

If the UK leaves the EU but Scotland or Northern Ireland wanted to join the EU then they would first have to become States (or become part of other States) to be able to join the EU by virtue of the largely forgotten Article 49 of the TEU.

The negotiations following the triggering of Article 50 will be long and tense. This is the style of many EU negotiations. The French walked out of negotiations for months during the “Empty Chair” Crisis in the mid-1960s. The Greek issue is still with us. Thatcher’s UK Rebate in 1985 took years of painful and fractious negotiation. The EU Merger Control Regulation took 17 years to agree. The euro took about 30 years to come to fruition. And to cap it off, the departure of a Big State – indeed, any State – is entirely unprecedented stuff.

The negotiations will be late and troublesome. The EU rarely reaches agreement on time - not to mind, ahead of time. The fisheries talks prove that. We could be facing a “EU law is switched off in the UK at midnight”-type moment. Experience from the Grexit, EMU Crisis, Financial Crisis and other negotiations demonstrates that one has to grip the steering wheel and keep going.

The negotiations will be tough. The interests differ radically between the parties. The talks cover the most difficult of issues including money, sovereignty, pride, trade and people. The UK wants to concentrate on trade but most of the 27 prioritise the EU vision, people, money, certain national interests and, as part of the mix, trade. Even the "divorce bill" estimated by some EU sources at up to €60 billion but the UK reckons is much less will be a flashpoint.

The challenge in the possible EU deal of 2019 deal is to avoid the Versailles Treaty of 1919 problem of concluding a treaty which is unduly tough on one State – it was Germany in 1919 and it should not be the UK in 2019 - but equally, the EU will not want to make the EU too easy to leave or to enable those who leave to prosper better than those who remain.

Depending on the arrangement concluded, there is a possibility that Ireland may even need a referendum which means that the UK’s and the EU's future could be partially in Irish hands.

Triggering Article 50 was the easy bit: the EU is like a lobster pot – easy enough to enter but difficult to leave.