Taking the UK out of the EU single market would be “the greatest job-killing act in Welsh economic history”, Plaid Cymru has said.
Several of Sunday’s newspapers claim Prime Minister Theresa May will signal the move in a speech on Tuesday.
Plaid’s treasury spokesman Jonathan Edwards told the BBC’s Sunday Politics Wales programme the impact on Wales would be “devastating”.
Downing Street has described the reports as “speculation”.
The Carmarthen East and Dinefwr MP said pulling out of the single market and customs union would have a “huge impact on jobs and wages in Wales”.
“The reality of what we’re going to hear from [Theresa May] on Tuesday, it’s going to be the greatest job-killing act in Welsh economic history, probably in British economic history,” he added.
“In that regard my colleagues and I are going to be opposing this extreme Tory government every step of the way.”
The Conservative AM for Monmouth, Nick Ramsay, told the programme Mrs May would ensure the best bespoke deal for the economy.
“There are advantages of being in the market but there are also disadvantages and these are very complex negotiations that have been ongoing,” he said.
“At the end of the day, we need the best deal for Wales, the best deal for the UK. And I’m confident that the team are trying to achieve that.”