The six-page letter informed him that the UK wishes to leave the EU and that the task ahead will be momentous.

“We are leaving the European Union but we are not leaving Europe – we want to remain committed partners and allies to our friends across the continent,” said prime minister Theresa May.

The triggering of Article 50 of the Lisbon Treaty on Wednesday in Brussels means the two-year countdown to the UK leaving the EU is now on.

Speaking in the House of Commons moments after the delivery of the letter, prime minister Theresa May said: “The Article 50 process is now underway and, in accordance with the wishes of the British people, the UK is leaving the European Union.

“This is a historic moment from which there can be no turning back. Britain is leaving the European Union. We are going to make our own decisions and our own laws. We are going to take control of the things that matter most to us and we are going to take this opportunity to build a stronger, fairer Britain.”

The Article 50 letter contained specific reference to Ireland, with Theresa May saying she wanted to make sure there was no hard border and that Brexit did not harm the Republic.

In a statement, the European Council said: “We regret that the United Kingdom will leave the European Union, but we are ready for the process that we now will have to follow.

“For the European Union, the first step will now be the adoption of guidelines for the negotiations by the European Council. These guidelines will set out the overall positions and principles in light of which the Union, represented by the European Commission, will negotiate with the United Kingdom.

“In these negotiations, the Union will act as one and preserve its interests. Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and member states.”

No sector faces a greater threat from Brexit

Donald Tusk said there was no pretending this was a happy day and he stressed that as for now nothing has changed and EU law still applies to and within the UK

On Friday he will share a proposal of the negotiating guidelines with the member states to be adopted by the European Council on the 29 April.

The formal negotiations will soon be underway and the IFA president said farming and food will have to be centre stage in the Irish Government’s position.

“The implications are very serious for our largest indigenous industry, given the significance of the UK market for our food exports. No sector faces a greater threat from Brexit and we expect a strong negotiating stance by our political leaders to achieve the best outcome for Irish farming.

“The IFA wants to maintain the closest possible trading relationship between the UK and EU, while preserving the value of the UK market – and a strong CAP budget following the UK’s departure, which is critical for farm incomes, farm output and economic activity in rural Ireland.”

Ensuring Irish trade with the UK

Rowena Dwyer, IFA chief economist, added: “The implications of a hard Brexit are stark: the ESRI estimates a potential reduction of EU trade to the UK of over 60% for dairy and 85% for meat. Translating this to an Irish context would mean a fall of €1.5bn in meat exports, with dairy exports falling by over €600m.”

Cormac Healy, director of Meat Industry Ireland (MII), stressed the need for business certainty as the process begins.

“56% of our meat exports, accounting for in excess of €2bn annually, are sold into the UK market. That is why the outcome of these negotiations is so important. A hard Brexit will result in massive trade disruption and job losses, whereas a sensible outcome aimed at maintaining trading continuity will be beneficial to both the Irish and UK agri food sectors.”

He also called on the Government to ensure future trading relations with the UK are a top priority

“A realistic transition period leading to a negotiated free-trade agreement with the UK must be delivered to avoid damaging cliff-edge scenarios and to ensure business certainty in the interim,” Healy said.

We have to take responsibility for our interests

Meanwhile, John Comer, ICMSA president, said: “If ever there was a time when we must look to our own national interests, then this is it.

“Such is the extent of Ireland’s exposure to a hard Brexit, we can not afford to adopt a wait-and-see attitude regarding talks”, he said.

“We have to take responsibility for our interests. It’s impossible to overstate how crucial it is that Ireland is aware of and party to the shape of these talks from the get-go."

General secretary of ICSA Eddie Punch warned that Irish agriculture did not cause Brexit so it cannot be the loser from it.

"The Government must ensure that the guidelines given to Michel Barnier make tariff free trade between Ireland and the UK a line in the stand. The Taoiseach cannot compromise on this with the other EU leaders."

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Brexit: what happens now?

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