Brexit: Carwyn Jones questions reported Irish border deal
The whole of the UK should stay in Europe's customs union, Wales' First Minister Carwyn Jones has said.
He spoke as Theresa May confirmed the UK and EU had yet to strike a deal to move to the next stage of Brexit talks.
But a Belgian MEP claimed the UK was ready to accept Northern Ireland may remain in the customs union and single market "in all but name".
Mr Jones said: "We cannot allow different parts of the UK to be more favourably treated than others."
Visiting Brussels for talks on Monday, Mrs May said progress had been made but differences remained on a "couple of issues".
The other 27 EU nations have said progress must be made on the Irish border, the so-called "divorce bill" and the rights of EU nationals before Brexit talks can move to the future trading relationship between the UK and the bloc.
Responding to reports about possible concessions on the Irish border, the first minister told BBC Wales: "For me the way to deal with this is to say 'look, the UK will stay in the customs union - and that means of course the border can remain as it is'."
It was a mistake for the prime minister to rule out keeping the whole of the UK in the single market early in the process, Mr Jones said.
"At the time of the referendum nobody talked about the customs union at all," he said.
Using Norway as a template - a country that is in the single market but outside the EU - "makes perfect sense", Mr Jones added.
Writing earlier on Twitter, Mr Jones said: "We cannot allow different parts of the UK to be more favourably treated than others.
"If one part of the UK is granted continued participation in the Single Market & Customs Union, then we fully expect to be made the same offer."
Plaid Cymru has also said "any special deals" made available to one part of the UK must also be available to others, including Wales.
The party said any deal to "shift the hard border to Wales' coast" could be "catastrophic" for Welsh ports like Holyhead.
Brian Thomas, deputy president of the Farmers' Union of Wales, said they had "a real sense of worry, alarm and anger" about the state of Brexit negotiations.
"Whilst we recognise that the Irish border situation presents some unique challenges that need to be overcome, rural Wales has challenges of its own", he said, pointing to its "heavy reliance" on red meat exports to the EU.
BBC political editor Laura Kuenssberg said she had been told by Belgian MEP Philippe Lamberts that the UK had made a concession on the Irish border.
She said Mr Lamberts had said the UK was prepared to accept that Northern Ireland may remain in the EU's customs union and single market in all but name.
But, she stressed, the BBC has not seen the draft document nor had it been signed off.
However, Democratic Unionist leader Arlene Foster - whose party has a deal to keep the Conservatives in power at Westminster - has said they "will not accept any form of regulatory divergence" that separates Northern Ireland from the rest of the UK.
She went on to accuse the Republic of Ireland of "seeking to unilaterally change" the Good Friday Agreement - the peace deal that brought an end to the Troubles - without the DUP's "input or our consent".
"Of course we will not stand for that," she added.
Scottish First Minister Nicola Sturgeon reacted to reports that Northern Ireland could retain "regulatory alignment" with the EU by saying there was "surely no good practical reason" why other parts of the UK could not do the same.