"Don't blink," Sinn Féin's leader Mary Lou McDonald told EU chief negotiator Michel Barnier at a meeting in Brussels on Wednesday.
The message urging continued, with unflinching commitment to the principles of the Northern Ireland backstop a familiar one, Ms McDonald admitted, as she paid tribute to Mr Barnier's and fellow member states' "genuine" and "steadfast" commitment to protecting the Belfast Agreement.
She told Mr Barnier that Sinn Féin was open to EU "assurances" to the UK to assist British prime minister Theresa May to get the vote through the Commons, but these "cannot intrude on the substance of the backstop".
If what they seek is to undermine or sidestep the Withdrawal Agreement protocol on Northern Ireland, "it cannot happen".
In their discussion of the possibility of a no-deal Brexit, the Sinn Féin leader said that if the UK "crashes out, there will inevitably be a hardening of the border" whether in terms of infrastructure or the less tangible aspects of the Border. In those circumstances "it will be imperative to put the issue of the Border to the people of Ireland by referendum".
The next logical step, she said, was for the Government in Dublin to start preparations for a referendum on unity. Even the British government appeared to accept that logic, she said, citing unconfirmed reports that Northern Ireland Secretary Karen Bradley had admitted to the British cabinet that a poll would almost inevitably have to be called in the event of a no-deal scenario.
The Sinn Féin leader also met members of the European Parliament steering group on Brexit.
