Any future deal on Brexit should be ratified by all four UK parliaments, First Minister Carwyn Jones has said.
He spoke after a meeting of the British Irish Council in Cardiff to discuss the UK's vote to leave the European Union.
Mr Jones said Brexit was the biggest challenge the administrations gathered at the council have faced collectively.
The UK government's Northern Ireland secretary James Brokenshire insisted that Theresa May's administration was "in listening mode".
"My view is that any future deal the UK agrees should be ratified by all four parliaments within the UK in order to get the greatest buy-in," Mr Jones said.
He said they could not be "mere consultees" and had to be "very much part of that negotiation".
Wales' first minister was joined at the "extraordinary summit" by his counterparts from Scotland and Northern Ireland, Irish Prime Minister Enda Kenny and government representatives from Jersey, Guernsey and the Isle of Man.
The special session of the council was called by Mr Jones to discuss the implications of the Brexit vote, including replacing EU funding and questions over the land border between the UK and continuing EU member Ireland.
The council usually meets annually, last convening in Glasgow in June.
Addressing a news conference after the meeting, on Friday, Mr Jones said there would be "fundamental changes" as a result of the EU referendum, adding: "During this tumultuous time, it is more important than ever to maintain the strength of this relationship and work together to map out a successful way forward."
He said the session had been "hugely helpful" in identifying challenges, with the council resolving to work together to find solutions.
Scotland's First Minister Nicola Sturgeon said there had been a "frank and very robust" discussion at the meeting, with her focus on ensuring Scotland plays a full part in the Brexit discussions.
She also questioned the process by which the decision to trigger Article 50 - the formal process of leaving the European Union - will be taken.
Northern Ireland Deputy First Minister Martin McGuinness said he had huge concerns about the prospect of a hard border with Ireland, and that Friday's meeting was the most important British Irish Council meeting he had attended.
For the UK government, Northern Ireland secretary Mr Brokenshire repeated the prime minister's line that "Brexit means Brexit".
He said: "I do not want to see a return to the borders of the past."
On the subject of the Irish border, Mr Kenny said: "It will not be acceptable, either south or north, that there will be a European Union border between Dundalk and Derry."