Theresa May should “come clean” on Brexit and put the national interest ahead of her party, Wales’ first minister will say later.
Carwyn Jones will urge the prime minister to “erase the wobbly red lines” and negotiate positively with the European Union.
He is making a speech in London to mark two years since the referendum.
UK ministers have faced two days of Commons votes on the EU (Withdrawal) Bill, overturning Lords amendments.
The first minister will call on Mrs May to use her government’s forthcoming Brexit White Paper to “come clean on Brexit, to erase the wobbly red lines and to get the country into a proper Brexit-ready position”.
Warning of the “very real possibility of a catastrophic no-deal Brexit”, Mr Jones will say UK ministers should “argue for a dynamic and positive relationship with the [European] Single Market”.
“Rather than acting as cheerleaders for populist attacks on the core principles of the EU, we need to provide reassurance that we respect the rights of the EU to safeguard the coherence of the Single Market which we did so much to create,” he will say.
Calling Brexit the “greatest challenge in peacetime to this country for generations”, Mr Jones will urge the prime minister to put the UK’s needs first ahead of Conservative party divisions.
He will argue Mrs May should challenge European Union chief Brexit negotiator Michel Barnier to “make good on their promises that it is our red lines that prevent a very different future partnership”, urging her to “be led by the national interest, not by fear of a few dozen backbenchers”.
Last week, Mr Jones was critical of “intense public argument between members of the government” following reports that Brexit Secretary David Davis had threatened to resign.
However, Labour has its own divisions, with dozens of MPs defying leader Jeremy Corbyn to abstain on a measure to keep the UK in the Single Market through membership of the European Economic Area (EEA).