First Minister Carwyn Jones has accused new UK government ministers of being naïve over future relationships with the European Union.
The Welsh Labour leader said Whitehall had to "re-learn the art of diplomacy" rather than "shouting at foreigners" to secure trade deals after Brexit.
Welsh voters said they wanted the UK to leave the EU in June's referendum.
The UK Government said it will get the "best possible deal for the whole of the United Kingdom as we leave the EU".
Mr Jones, the longest serving current leader of a UK nation, told the BBC's Sunday Politics Wales show: "There are some who are realists, but there are others who still say Europe will, basically fall at our feet," he said.
"They're not going to do that; they have to be realistic and understand this is a discussion between not just two parties but more than 30 different parties when you take into account the different parliaments in the EU as well.
"The UK needs to re-learn the art of diplomacy rather than shouting at foreigners, which is the impression you get from some of the Tory politicians."
Mr Jones has led the Welsh Government for seven years, longer than current Scottish First Minister Nicola Sturgeon, Northern Ireland's Arlene Foster, and UK Prime Minister Theresa May.
Earlier, Welsh Secretary Alun Cairns said Brexit would help communities who had previously missed out from EU funding.
Over the last 16 years, more than £4bn in EU aid has been spent in west Wales and the valleys - the areas which have benefitted the most.
But the Vale of Glamorgan MP told BBC Radio Wales' Sunday Supplement programme leaving the EU would provide the opportunity to reshape how structural funds are shared.
He said areas such as parts of Barry, which is in his constituency, or areas across Powys were in desperate need of support.
"Because of the European map as it's drawn, they haven't been able to draw on the same levels of EU funding that parts of the traditional west Wales and the valleys have," he said.
Mr Cairns stressed no decisions had yet been made on structural funds or repatriated powers once the UK leaves the EU but there was a "positive opportunity" to support the poorest communities.
He added: "How many times have we heard from some of the charities or local authorities that draw down the funding, the talk about the frustration behind the European rules that doesn't allow them to do certain things?
"Well, we can now set UK priorities to benefit those communities as they see fit rather than as some bureaucrats in Brussels see fit."
Prime Minister Theresa May intends to trigger the process for the UK to leave the EU by the end of March 2017 and will then have two years to complete it.
That remains the plan despite Supreme Court judges still considering whether or not to overturn a High Court ruling stating Parliament must have a say in the Brexit process.
In the four-day hearing earlier this month, Welsh ministers joined campaigners and the Scottish Government in challenging the UK government's belief that it could trigger the process of leaving the EU without MPs' authorisation.
A Department for Exiting the EU spokesperson said: "As the prime minister has said, we are determined to get the best possible deal for the whole of the United Kingdom as we leave the EU.
"We have also been clear that we're seeking a new partnership with the EU that works in the mutual interest of both sides, and are approaching these negotiations with a spirit of goodwill.
"We are working closely with the Welsh government and the other devolved administrations to ensure we make a success of Brexit."