Sinn Fein president Mary Lou McDonald said it was “very concerning” that Theresa May had not removed the option of a no-deal from the table.
Ms McDonald also said it was “absolutely essential” that cool heads prevailed and the Irish Government did not bow to pressure from its British counterpart.
“I do think it’s concerning that the clock is ticking and although there’s a parliamentary majority [in Westminster] that says we don’t want a hard border, we don’t want a crash, that the Prime Minister still is not willing to officially take that option off the table,” she said.
“I think that’s extremely unhelpful, I think that raises tensions. It raises concerns.”
Ms McDonald made the comments outside Leinster House on Tuesday.
She added that there was no room for negotiation on the existing deal.
“The negotiation on the Withdrawal Agreement and the backstop is over,” she said.
“That passage of play is finished and there’s no room to go back and unpick those matters.”
Ms McDonald said that Irish Government needed to stand firm when it came to protecting the backstop.
“It is absolutely essential that cool heads prevail, that the Government here doesn’t blink, irrespective of what pressure tactics or what overtures are made from London,” she said.
But she said it was still important that there was sufficient contingency planning for a no-deal scenario.
Ms McDonald said she had been briefed by Ireland’s foreign affairs minister in respect of proposed legislation when it came to east-west contingency planning, but she said the Government’s response was “still pretty threadbare” when it came to north-south contingency planning.
Asked whether the resumption of powersharing at Stormont could be a solution to the issue, she said: “That actually wouldn’t be the answer to the Brexit dilemma.
“This Brexit negotiation is between European Union institutions and British government.”
She added: “I think it’s a disgrace that we don’t have operational government in the North, but I don’t want you to imagine for a second that that would be the silver bullet in this scenario. It frankly wouldn’t be.”
She said British parliamentarians were already aware that the majority of politicians in Northern Ireland did not support Brexit.
It is two years this month since the Assembly collapsed.
Ms McDonald said her party had done everything it could to get the powersharing institutions back up and running and that the DUP had walked away.
“Both governments, not just the British government, have acquiesced with the DUP to keep the institutions down,” she said.
“They have given way to that DUP agenda.”
Fianna Fail Brexit spokeswoman Lisa Chambers said she was disappointed that Mrs May had not provided a clear roadmap outlining how they want to proceed.
At this point Ms Chambers described the backstop as the “only show in town”.
“We’re dangerously close to a cliff edge and I am concerned about our own government’s preparations in the event of a no-deal Brexit,” Ms Chambers said.
She said France, Germany and the Netherlands had already passed its emergency legislation and that it was unacceptable that the Irish Government had not yet provided an outline of the legislation.
She said Fianna Fail could not understand the secrecy by the Government.