- Video report by ITV News National Editor Allegra Stratton
Theresa May has formally triggered the two-year process of the UK's withdrawal from the European Union.
The letter will be delivered to European Council President Donald Tusk on Wednesday, formally beginning the Brexit process.
But what happens now the Brexit starting gun has finally been fired?
- What is Article 50?
Article 50 forms part of the Lisbon Treaty which was signed by all members of the European Union in December 2007, and it sets out the rules for the process of a member state leaving the EU.
It starts a two-year period for Britain to negotiate terms of its withdrawal from and future relationship with the union.
- Will triggering Article 50 immediately affect the UK's EU membership?
The UK will remain bound by EU laws and regulations during the next two years and it must also continue to honour its commitments as a member state.
However, the UK will be excluded from affairs when the 27 other countries are discussing the withdrawal or internal EU business.
- Can Article 50 be revoked?
As no country has triggered Article 50 before there is no previous test case on whether it is reversible and Theresa May has previously emphasised that "Brexit means Brexit".
- What is the Brexit time-table?
The 27 other EU countries will meet to discuss Britain's withdrawal "setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the union".
Negotiations will then begin between the UK and EU.
After a draft deal is agreed with the European Commission it will be put to the European Council for approval.
It will then be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.
The UK will then repeal the European Communities Act 1972.
The government plans to introduce the "Great Repeal Bill" to enable EU regulations to be converted into domestic law, so parliament can pick and choose which laws should be kept or revoked.
- Who is going to negotiate the exit for Britain?
Three main Brexiteers, will play key roles in negotiations with the EU, they are: David Davis, Brexit Secretary, Liam Fox, International Trade Secretary and Boris Johnson, Foreign Secretary.
However, it will be the Prime Minister who will have the final say on any deal.
- And on the EU's side?
Former French minister Michel Barnier is the European Commission's chief Brexit negotiator.
He is said to be a hardliner and has previously said Britain will have to accept freedom of movement - "without exception or nuance" - if it wants to retain access to the single market."
Mr Barnier has also said the UK will have less than 18 months to negotiate and any deal must be finalised by October 2018.
He is reportedly seeking a 60 billion euro (£52 billion) "exit bill" from Britain.
- What key things need to be agreed ?
Access to the EU single market and trade deals for goods and services between Britain and the remaining member states will be top of the agenda.
Free movement and the rights of EU nationals already in the UK and the same assurances for Britons living abroad will also need to be clarified.
- How much will the divorce cost the UK?
The UK can also expect it to be a very costly departure.
European Commission President Jean-Claude Juncker has previously warned: "Our British friends need to know - and they know it already - that it will not be cut-price or zero-cost."
- What happens if no deal is agreed at the end of two years?
Many experts predict that it will be a difficult divorce which is likely to take more than two years to reach settlement.
The UK could potentially get an extension on the negotiating period but only if all EU members states agree.
- Could we leave without a deal?
Yes, the UK could leave without any agreement and existing treaties and EU law would cease to apply. However, there might be some acquired rights for EU and UK citizens.
This would also mean falling back on default World Trade Organisation tariffs which are higher than those we have currently.
Professor Anand Menon, from UK in a Changing Europe, told ITV News: "I'd say there's probably a 50% chance that we come out of this without a deal, simply because the politics of money in the European Union can get poisonous really quickly.
"There might well be a big gap between what we are thinking we want to pay and what the others are expecting and at that point it might be that Number 10 starts thinking maybe we should walk away."
- Would there be a financial penalty for no post-Brexit deal?
The UK would be in a "strong" position to walk away from the EU without paying anything if there is no post-Brexit deal, a House of Lords inquiry has concluded.
But it also cautioned against the UK walking away without making a financial contribution as this "would also damage the prospects of reaching friendly agreement on other issues".
- Can Britain rejoin after leaving?
If the UK decides in the future that it want to rejoin the EU, it would have to re-apply under Article 49 like any other country.