Gerry Adams has been accused of stoking fears about a return to violence in Northern Ireland during a Brexit speech.
Ulster Unionists have urged the Sinn Fein leader and Louth TD to clarify remarks he made that Brexit and London's indifference to its impact across Ireland could "destroy" the Good Friday Agreement.
In a weekend speech in Dublin, Mr Adams called on Taoiseach Enda Kenny to win support from the 26 other EU states for a 'special designated status' for Northern Ireland.
"Brexit is a hostile action, the British Government's decision to drag the North out of the EU against the wishes of the electorate is a hostile action," he said.
"This and the indifference of the Tory Government in London towards Ireland, north and south, will destroy the Good Friday Agreement."
UUP MLA Doug Beattie said the language Mr Adams used at an event in the Mansion House on a united Ireland could be seen as inflammatory and inciting violence.
"For him to say Brexit will destroy the Belfast Agreement will cause fear in the minds of many that we could return to the days before the Belfast Agreement, where violence was an everyday occurrence," he said.
"When these comments are added to the inflammatory language that Brexit can be viewed as a hostile act, he invites the notion that could be interpreted by some physical force republicans, that a hostile act could justifiably be countered by other hostile acts."
The Government also rejected Mr Adams assertion the 1998 Agreement could be 'destroyed' by the UK pulling out of the EU.
"None of the institutions and provisions set out in the (Good Friday) Agreement, including those relating to human rights, are in any way undermined by the decision of the UK to leave the EU," a statement said.
"The UK Government is fully behind the implementation of the Belfast Agreement and its successors, including the Stormont House Agreement and Fresh Start Agreement.
"There will be no return to the borders of the past. We are also working intensively to ensure that following the forthcoming election, strong and stable devolved government that works for everyone is re-established."
Meanwhile, a former head of the European Commission's customs procedures has claimed Irish border controls will return when Britain leaves the EU. Michael Lux told the BBC's Inside Business show that coloured lanes could be used as a form of customs control.
"So there would be two lanes for vehicles; a green one where you are only occasionally stopped and a red lane if you carry goods above the threshold," Mr Lux told the BBC's Inside Business programme.
"Such a system can be established between Northern Ireland and the Republic of Ireland."