MARY Lou McDonald has accused the DUP of a "reckless disregard" for the north's economy and political agreements, and urged it and the British government to "get real" on Brexit.
The Sinn Féin president said both the DUP and Tories should respect the vote of the majority in the north to remain within the European Union with all that entails.
Her comments come as Theresa May's British government faces a major setback in Brexit negotiations after European leaders rejected her Chequers proposals.
The two sides continue to disagree on how to prevent a hard Irish border when the UK formally leaves the European Union next year.
Ms McDonald said: "The events of the past week around the European summit on Brexit have demonstrated once again that Theresa May's sole interest is in clinging to power.
"While the European leaders have been seeking solutions to the Brexit problem the British government has moved to ultimatums and stand-offs."
The Dublin TD said the EU's 'backstop' proposal – which would see Northern Ireland effectively remaining within the EU single market and customs union if no other border agreement is reached – is a "common-sense solution to the British Brexit" that would "deliver for all our people north and south".
The DUP – on which the Tories rely for key Westminster votes – has supported Mrs May's stance, with Arlene Foster saying Mrs May was "right to stand firm in the face of disrespectful, intransigent and disgraceful behaviour by the European Union".
The DUP leader added that the UK would "not countenance any new regulatory or customs barriers" between Northern Ireland and Britain.
Ms McDonald, speaking on Saturday night at the installation dinner of Sinn Féin mayor Deirdre Hargey at Belfast City Hall, said: "It is clear the DUP has a reckless disregard for the economy, agreements, livelihoods and jobs of the north.
"They do not represent the views of the majority of people nor the interests of the economy.
"The days of unionist veto are gone and will not be replaced by a Tory veto.
"Both the Tory party and the DUP should respect the vote of the majority in the north to remain within the European Union with all that entails.
"The DUP cannot veto the vote of the people in the north to remain.
"It is time for the DUP and British government to get real. There will be no withdrawal agreement without a backstop that protects the interests of all Ireland."
In Northern Ireland 56 per cent backed Remain in 2016's EU referendum, but overall 52 per cent in the UK voted Leave.
Alliance deputy leader Stephen Farry, the party's Brexit spokesperson, yesterday called for a "de-escalation of the rhetoric against the backstop".
He said rhetoric around the backstop being a constitutional problem "is simply wrong".
"Northern Ireland already does many things differently through devolution, including aspects of economic policy. The backstop if ever implemented would only build upon these precedents," he said.
He added: "The UK government and opposition are risking a catastrophic no-deal Brexit based on a counter-productive and short-sighted political agenda, a disregard of the political and economic interests of Northern Ireland, and a misunderstanding of the agreement.
"The DUP are potentially dragging down not only Northern Ireland but the UK as a whole."
SDLP leader Colum Eastwood last week accused the British government of leaving the people of Northern Ireland "in an extremely vulnerable position".
"The people of Northern Ireland did not vote for this mess. What we did vote for was the Good Friday Agreement. Any diminution of that agreement will not be supported in Ireland or by the EU," he said.
