Taking Northern Ireland out of the EU will "destroy" the Good Friday Agreement peace deal, Gerry Adams said.
He claimed fundamental human rights enshrined in the 1998 accord to end violence could be undermined. The top legal adviser to Stormont ministers has said not one word in the Agreement would be affected.
The Sinn Fein president said Northern Ireland should enjoy special status within the union of 27 states after Brexit and claimed that would not affect the constitutional settlement which secures its status as part of the UK.
He said: "Taking the North out of the EU will.
"It will destroy the Good Friday Agreement."
The Dail TD (member of the Irish parliament) addressed a conference on achieving a united Ireland in Dublin.
Mr Adams added: "The British government's intention to take the North out of the EU, despite the wish of the people there to remain, is a hostile action.
"Not just because of the implications of a hard border on this island but also because of its negative impact on the Good Friday Agreement.
"The British Prime Minister repeated her intention to bring an end to the jurisdiction of the European Court.
"Along with her commitment to remove Britain from the European Convention on Human Rights this stand threatens to undermine the fundamental human rights elements of the Good Friday Agreement."
He claimed ending partition between Northern Ireland and the Republic had taken on a new importance.
"As the dire economic implications of Brexit take shape there is an opportunity to promote a new agreed Ireland."
Northern Ireland voted to remain in the EU referendum by a majority of 56% to 44%.
Mr Adams added: "The speech by Theresa May will have reinforced this.
"The dangers of a hard Brexit are now more obvious than before.
"The North needs a special designated status within the EU.
"The Irish government needs to adopt this as a strategic objective in its negotiations within the EU 27 as they negotiate with the British Prime Minister."
He claimed there was, alarmingly, no strategic plan from Dublin ministers.
The Irish Government has already convened an all-Ireland forum on Brexit and agreed with the Prime Minister that there should be no return to the borders of the past for Northern Ireland.
Its priorities remain its economic and trading arrangements, the peace process and border issues as well as the common travel area.
Mr Adams added: "The British position also fails to take account of the fact that citizens in the North, under the Agreement, have a right to Irish citizenship and therefore EU citizenship."