Simon Coveney, the Irish foreign minister, said that he was not willing to allow talks in Brussels to move on to trade and transition without more assurance that there would not be a return of border checks between the North and the Republic.
He claimed there was "a lot of solidarity" among the EU27 for its position that Northern Ireland must retain the regulatory framework of the single market and customs union in order to avoid creating a hard border with the Republic.
Yesterday Mr Coveney insisted there was a "lot of solidarity" in Brussels for demands that the United Kingdom provide greater assurances.
The Taoiseach said he had met Theresa May on Friday.
The EU is calling for a solution that would see Northern Ireland continue to adhere to the EU's regulatory framework, to enable the retention of a free-flowing border.
"This is a much bigger issue than trade".
She told the BBC: "Some people are taking their moment in the sun, to try and get the maximum in relation to the negotiations - and I understand that but you shouldn't play about with Northern Ireland particularly at a time when we're trying to bring about devolved government again".
DUP Westminster leader Nigel Dodds yesterday said the PM had assured him during a No.10 meeting that she would never agree to a border in the Irish Sea.
Mr Johnson did so, declaring: "There can be no hard border; that would be unthinkable". But MPs fighting against a hard Brexit warned that neither side had come up with a solution that could prevent a hard border if the United Kingdom insists on leaving the customs union and single market.
"It is therefore in Boris's hands, and those of his fellow ministers, to ensure there is no need for a hard border".
Sinn Féin Group leader on Derry City and Strabane District Council Councillor Paul Fleming is to bring forward a motion to the monthly council meeting on Thursday calling on the council to "write to each Head of Government of the European Union 27, including the Taoiseach, on behalf of the people of this border Council area who will be drastically affected by any hard border as a result of Brexit". He said: "The UK has shown outstanding patience and goodwill since serving the article 50 notice".
"And we are very serious on that because we do not want the relationship between Ireland and Britain, and in particular the relationships on the island of Ireland, to go backwards. Putting those arrangements in place will have the doubly beneficial effect of providing reassurance to business and signalling to the European Union that we are not disposed to be strung along".
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