Theresa May has promised to strengthen relations with ministers in Wales, Scotland and Northern Ireland ahead of a meeting of the UK's top politicians.
The joint ministerial council (JMC), which includes first minister Carwyn Jones and the prime minister, meets on Monday for the first time since 2014.
Mrs May hopes it will be the start of a "new grown-up relationship".
Mr Jones said a stronger partnership was needed after the decision to leave the EU.
He told BBC Wales the UK government had not realised the scale of the challenge of reaching an agreement between the devolved nations over a future Brexit deal.
Mr Jones and Mrs May will be joined by Scottish First Minister Nicola Sturgeon and the First Minister and Deputy First Minister of Northern Ireland, Arlene Foster and Martin McGuinness, as well as Welsh Secretary Alun Cairns on Monday.
The council last met in December 2014.
The UK government said Mrs May would seek agreement on strengthening ways of working between the UK and devolved governments, guaranteeing relationships will be built on a basis of "co-operation and consensus".
Mrs May said: "I want Monday's meeting to be the start of a new grown-up relationship between the devolved administrations and the UK government - one in which we all work together to forge the future for everyone in the United Kingdom."
The prime minister is also proposing a series of changes for the JMC, with meetings taking place at least once every 12 months and committing to an annual report of the committee's work.
Under the shake-up, the venue of the JMC, which will be chaired in Downing Street by Mrs May on Monday, would also rotate between each of the four governments on a revolving basis.
Welsh Secretary Alun Cairns said the prime minister had set a "clear framework" for Wales to be at the centre of any Brexit negotiations.
The first minister said there was "nothing new" in the JMC changes beyond those already agreed for some time, but he welcomed Mrs May's "warm words" ahead of Monday's meeting.
Mr Jones told the Sunday Politics programme there needed to be "proper discussions" to reach a "common UK viewpoint".
He said it would make no sense to push forward with a Brexit negotiation that Scotland, Northern Ireland and Wales had not agreed on.
"If Whitehall can't get agreement with the devolved administrations, what hope has it got of getting agreement with 27 member states," he added.
A Welsh Government spokesperson said the UK's exit from the EU raised profound issues for all four governments.
"We must work together in a radically different way from before if we are to rise to the challenge we face and ensure the needs of all parts of the UK are properly addressed.
"The first minister looks forward to setting out our own vision for the future at Monday's JMC," they added.