Welsh and Scottish ministers have claimed they are being denied a chance to influence the Brexit negotiations.
They demanded to see draft copies of the UK government's Brexit white paper at talks in London on Thursday.
Wales' Finance Secretary Mark Drakeford said the meeting was a "useful" opportunity to press the case for a "jobs first" deal with the EU.
It comes a day before Theresa May's cabinet meets to agree what future relationship it wants with the union.
The prime minister will try to get her cabinet to sign up to the policy document on withdrawing from the EU at Chequers on Friday.
In a joint statement, Mr Drakeford and Scotland's Constitution Secretary Mike Russell said they had not been allowed to see a draft copy of the document "so it will be impossible for us to make the sort of considered contribution, based on evidence, which we think is essential".
They called it a "clear breach" of a commitment to "try to seek agreement on the negotiation position of the UK government".
After the meeting, Mr Drakeford said: "We have had sight of some small extracts from some chapters of the white paper.
"It is impossible for us to provide the UK government with a considered response in which we are able to inform the position they are taking without having seen the text at all."
Cabinet Office Minister David Lidington, who chaired the joint ministerial council meeting, said the UK government wanted to "engage constructively" with the devolved administrations.
He said: "It is important that the UK government sits down regularly with the devolved administrations to discuss our preparations for leaving the EU.
"We are determined to get the best possible Brexit deal for all parts of the UK."
Plaid Cymru's Brexit spokesperson Hywel Williams MP said: "It's taken a full six weeks for Mark Drakeford to realise how foolish and irresponsible his Labour government was to cave in to the Tories on their power grab of returning European powers. Now he moans about a 'clear breach' of the agreement.
"We warned Labour at the time that they were ludicrously naive to trust the Westminster Tories and that handing over Wales' powers and leverage in Brexit negotiations was a grave mistake which they would soon regret."