Taking Northern Ireland out of the EU will "destroy" the Good Friday Agreement, Gerry Adams has claimed.
Speaking at a conference in Dublin on Saturday the Sinn Féin president claimed it would be a "hostile action".
The top legal adviser to Stormont ministers, however, has said the agreement will not be affected.
A government spokesperson said his comments were "totally without any basis in fact" and there would be "no return to the borders of the past."
Outlining plans for the UK leaving the EU on Tuesday, Prime Minister Theresa May said Brexit means leaving the European Customs Union.
Mr Adams said Northern Ireland should enjoy special status after Brexit, claiming it would not affect the constitutional settlement which secures its status as part of the UK.
"Taking the North out of the EU will destroy the Good Friday Agreement," he said.
"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."
On Tuesday, the Taoiseach (Irish Prime Minister) Enda Kenny re-iterated his wish for "a preservation of the seamless border that is there now" and welcomed the "greater clarity" provided by Mrs May in her address on Britain's planned approach to the Brexit negotiation process.
Mr Adams 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 British position 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.
'No return to the borders of the past'
Following the comments a UK Government spokesperson said:
"These comments are totally without any basis in fact. None of the institutions and provisions set out in the Belfast Agreement, including those relating to human rights, are in any way undermined by the decision of the UK to leave the EU."
"The UK Government is fully behind the implementation of the Belfast Agreement and its successors, including Stormont House and Fresh Start.
"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 in Northern Ireland."