In Northern Ireland a free-flowing border with the Republic allows many industries to flourish - so how will this be affected by a so-called "hard Brexit?"
ITV News Correspondent Martin Geissler reports from the Irish Border, where two business managers have opposing views on the impact of border posts and tariffs:
The former head of the Diplomatic Service breaks down the hidden meanings behind Theresa May's letter to Donald TuskRead the full story ›
Nine months on, Stratford - where the referendum vote was 52:48 - remains as divided as ever over whether leaving the EU is a good idea.Read the full story ›
EU negotiators believe a trade deal will have to follow a Brexit deal - which could cause a bust-up over the negotiation timeline.
Officials think Britain should lay out its exit before discussing its future links with current member states.
- ITV News Political Correspondent Carl Dinnen reports from Brussels
Nigel Farage has celebrated the triggering of Article 50 by going to the pub for a pint.
The former UKIP leader said it was a "big day" and he was having a "minor celebration".
He added: "Every day is a sad day in the European Union because every single day that goes by opposition to the project grows amongst all 28 member states.
"The European Union may have been a good idea 60 years ago. It doesn't work anymore."
The chief Brexit representative for the European Parliament has said that the basic principle driving Britain and the EU's new relationship should be to put "citizens first".
European negotiator Guy Verhofstad told a news conference the parliament was "completely against citizens becoming bargaining chips in negotiations."
"When we talk about citizens, it's citizens in Britain and citizens in the European Union," he said. "It's an absolute priority that the rights of these citizens is settled."
We want the EU-27 to examine hot to mitigate the negative effects for those British people who are losing their European citizenship.
Verhofstad said that the EU and Britain should agree on a general framework of a new relationship withing two years. Britain can change its mind on leaving the EU - if all other EU states agreed to its return, he added.
President of the European Parliament Antonio Tajani has said that a "no deal" Brexit scenario would be a "catastrophe for all - but especially the United Kingdom."
"The UK would be faced with tariffs, uncertainty for the car industry and financial services, increased food prices, high traffic disruptions, long lines of lorries in Dover to name just a few," he told a news conference.
The veteran Italian politician said the UK will have to fully respect its treaty obligations until the last day of membership, including its financial commitments.
"The UK will not be asked to pay for anything they had not previously agreed to," he added.
Tajani described Wednesday as a "bad day" after Britain informed the European Council of its intention to leave the European Union.
The key European Union Brexit negotiator with the UK has said his team is "ready".
Michel Barnier also tweeted a photograph of his team who he said would "work for the 27 European Union members" during Brexit negotiations.
ITV News' Security Editor Rohit Kachroo said the letter from Theresa May triggering Brexit negotiations suggests the UK will focus on security as a negotiating tactic.
"Alongside economy, frequent mentions of security and terror in PM's article 50 letter," he said. "Clearly, UK will use its security clout as leverage."
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