UK ministers must make clear what Brexit deal they want "pretty soon" to ensure a "reasonably united front" with devolved nations for the negotiations, First Minister Carwyn Jones has warned.
He said the UK government would not want criticism from Wales, Scotland and Northern Ireland before the talks.
Mr Jones is on a three-day visit to Norway to learn about its EU links.
In December, Prime Minister Theresa May pledged to set out her Brexit proposals in a speech in the new year.
She has said the UK will begin the formal Brexit negotiation process by the end of March.
On Wednesday, Mr Jones told BBC Radio Wales: "We want to be in a position where we have agreed a line with the whole of the UK, before they [the UK government] go into negotiations.
"The last thing they will want is for Scotland, Wales and Northern Ireland to be publicly critical of them as they enter those negotiations.
"Now that's not what I want to do, let's get that clear, that's not my starting point, but they will want to avoid that."
UK ministers would want a "reasonably united front in order to go into those negotiations with what I would want to see, which is our support", Mr Jones said.
"So far we don't know what their position is, we have no idea what they're thinking, and that needs to be made clear pretty soon, for everybody's sake," he added.
Before setting off for Oslo, Mr Jones repeated his call for Wales to maintain "full and unfettered access" to the European single market.
Although not an EU member, Norway pays for full access to the European single market, accepts most EU laws and allows EU citizens to live and work there.
Norway is exempt from EU rules on agriculture, fisheries, justice and home affairs but has no say over how the rules of the single market are created.
He said "the Norwegian model is not ideal" but the trip gave him opportunity "to look at what's best for Wales and Britain".
In November, the first minister told AMs that maintaining access to Europe's single market without extra costs was a more important issue than immigration.
On Wednesday, Welsh Conservative leader Andrew RT Davies accused Mr Jones of seeking to "have his cake and eat it" by demanding full single market access while accepting the need for immigration controls.