Gerry Adams has said the deal between the Conservatives and Northern Ireland's DUP "provides a blank cheque for a Tory Brexit which threatens the Good Friday Agreement."
The President of Sinn Fein previously said he told Theresa May she is in breach of the Agreement when he met the Prime Minister at 10 Downing Street.
In a statement to reporters outside afterwards, Mr Adams said: "We told her very directly that she was in breach of the Good Friday Agreement, and we itemised those matters in which she was in default in relation to that agreement."
He also confirmed they had also discussed the possibility of a referendum on Irish unity.
In a statement released after Ms May secured a deal with the DUP to prop up her Government, Mr Adams said: "The price of today's DUP-Tory deal is DUP support for continued Tory Austerity and cuts to public services.
"It provides a blank cheque for a Tory Brexit which threatens the Good Friday Agreement."
He added: "Sinn Fein will vigorously pursue the rights of citizens currently being denied by the DUP and the British government.
"We are committed to equality. Sinn Fein will resolutely oppose any attempt to give preferential treatment to British forces, either in terms of legacy or the provision of public services.
“If as they claim in today’s agreement, both the Tories and the DUP will fully adhere fully to the Good Friday Agreement and its successors, they need to deliver on this for the political institutions can be restored.
“So there is work to be done by the DUP and only limited time to do this. As they return to Ireland to meet with Sinn Féin and the other parties, the DUP should be minded of the words of Edward Carson speaking in 1921 on the Tory intrigues that had led him on a course that would partition Ireland: ‘What a fool I was. I was only a puppet, and so was Ulster, and so was Ireland, in that political game that was to get the Conservative party into power’.”
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