The first minister has called on Theresa May to rethink plans for how the UK should leave the European Union.
In a letter to the prime minister, Carwyn Jones claimed the general election result showed there was “no mandate” for a “hard Brexit”.
He called for an urgent meeting of the joint ministerial committee (JMC) of UK leaders to establish a “broad-based consensus” on Brexit.
Welsh Secretary Alun Cairns said the government’s focus would be on trade.
In the letter, Mr Jones said it was “increasingly clear” a transitional deal was needed.
The first minister highlighted a UK government White Paper proposal to leave the customs union as an example of a measure that did not have widespread support.
“I hope you will recognise that, given the outcome of the general election, your government has no mandate for the sort of ‘hard Brexit’ which you have championed,” he wrote.
“By contrast, it is of critical importance to build a broad-based consensus across the UK and across parties and civil society about how to take forward the Brexit process.
“The JMC is, for the present, a fundamental part of that process.”
He added: “It is also increasingly clear that, given the political situation, it will be impossible to both negotiate a withdrawal agreement and put in place the basis for a future relationship with the EU within two years.
“It means we must agree within the UK now, and seek the agreement of our EU partners early in negotiations, the form of transitional arrangements to come into effect in April 2019.”
Mr Jones claimed the Welsh Government’s own White Paper – calling for continued “full and unfettered access” to the single market – “might form the basis of a UK wide negotiating position that could command considerable support”.
The first minister also urged Mrs May to pay “full respect to the existing devolution settlements” and warned against any imposition of new arrangements to handle matters such as agriculture and economic development when EU responsibilities in such areas ended.
“Bluntly, this is a fight which you do not need to have,” he said.
Speaking on BBC Radio Wales’ Good Morning Wales programme Mr Jones denied that the tone of the letter was threatening.
“Far from it. We will not accept anything that reduces the powers of Wales, Scotland and of Northern Ireland.”
“You have to be firm with them,” he added. “Scots have done exactly the same. It’s absolutely important that we make our positions very very clear.”
In response to Mr Jones’s letter, newly reappointed Mr Cairns said the UK government’s focus would be on “the right outcome” which was to achieve “frictionless trade” with the EU.
A spokesman for Welsh Conservative leader Andrew RT Davies accused Mr Jones of “cynical posturing” who “steadfastly refused to engage with Leave campaigners offering to work together after the referendum”.
The spokesman accused the first minister of attempting to “rewrite history”, saying both Labour and the Conservatives went into the general election pledging to leave the single market.
“There is now an opportunity to work with all parties to progress Brexit, but voters will not stand for attempts to change the referendum result,” he added.
Negotiations with Brussels on the UK’s departure from the EU are due to start on 19 June, and Mrs May is now seeking the backing of Northern Ireland’s Democratic Unionist Party (DUP) to prop up her minority government.
The DUP supports Brexit – but also wants to avoid a hard border with the Republic of Ireland and to maintain as far as possible the current access to EU markets.