Theresa May will invoke Article 50 of the Lisbon Treaty, beginning the separation process.
She has already signed the letter invoking Article 50 and it will be later hand delivered to European Council President Donald Tusk later.
Around the same time the British Prime Minister will address parliament on the issue.
After that the clock starts ticking on two years of intense and historic negotiations on what the Brexit deal will be.
Tomorrow the EU's negotiating guidelines will be given to Taoiseach Enda Kenny and the leaders of the 27 remaining member states, laying the ground rules for the fight to come.
The government have said they'll be pushing for a special deal for Ireland – taking into account the huge history between the two states.
The future of the Good Friday Agreement, the border with Northern Ireland and our trade links with the UK will all be top priorities for Enda Kenny, as the negotiations finally begin.
