SDLP leader Colum Eastwood has written to the first ministers in Scotland and Wales claiming Theresa May is refusing to engage with Northern Ireland's nationalist voices on Brexit.
The letter to SNP leader Nicola Sturgeon and her Welsh Labour counterpart Carwyn Jones was prompted by the British prime minister's remarks at Westminster on Monday, when she said she was "listening to unionists across the country, including the DUP".
Mrs May was responding to last Friday's EU-UK agreement on phase one of the Brexit negotiations.
Mr Eastwood is seeking support from Britain's devolved administrations for a place at the Joint Ministerial Council for Stormont's main parties.
The SDLP leader said Mrs May's statement did not "point to any serious engagement with parties in Northern Ireland which are not unionist".
"This partisan approach fails to understand the basic arithmetic of the Northern Ireland Assembly but more importantly, it is fundamentally at odds with the political dispensation of the Good Friday Agreement and must be challenged," he said.
“Unionism cannot call the shots alone, in the same way that nationalism cannot – any attempt to assert the dominance of one community or political party over another is a violation of the principles of partnership and power sharing."
The Foyle MLA said he was seeking support from the Scottish and Welsh first ministers for those parties who could potentially form a Stormont executive to be given a place at the Joint Ministerial Council.
He said Mrs May needed to "understand that the DUP does not speak for people in Northern Ireland".
"As the Brexit negotiations continue, the plurality of voices from Northern Ireland must be heard," he said.
