The government must consult Parliament before it can implement Brexit, the High Court has ruled.
It was decided that Prime Minister Theresa May cannot act alone in triggering Article 50 of the Lisbon Treaty, something which would have signalled the beginning of formal negotiations with the EU.
Mrs May had claimed that she could enforce the Article on the basis of royal prerogative, meaning she would not have to consult MPs.
But the High Court found in favour of the claimants – businesswoman Gina Miller and hairdresser Deir dos Santos.
The Prime Minister is expected to appeal the decision, with the Supreme Court suggesting early December as a time to hear the case.
She might not want to risk losing again, but could decide to go for it on the basis that MPs would not wish to defy voters who ultimately chose to leave the EU.
The pound jumped one per cent against the dollar upon the announcement of the ruling.
It follows yesterday’s All Island Civic Dialogue on Brexit – a meeting involving Irish politicians from both sides of the border – where Taoiseach Enda Kenny warned that talks between Britain and the EU could turn “vicious”.
“Neither I nor the prime minister desire to limit the freedom of people on both sides of the Irish sea to trade, live, work and travel freely across these islands.
“Therefore we have agreed that the benefits of the common travel area be preserved,” he said.
“The other side of this argument may well get quite vicious after a while, because there are those around the European table who take a very poor view of the fact that Britain decided to leave.”
He explained how Brexit is the greatest challenge facing Ireland since it gained independence from Britain.
Ireland’s foreign minister, Charlie Flanagan, who led the talks alongside Mr Kenny, reiterated the point that the Irish government would push for the retention of an “invisible” border between North and South.