A move to roll back devolution by the UK Government in Brexit legislation would be "anti-Welsh in the extreme", according to a Labour front-bencher.
Shadow Brexit Minister Jenny Chapman made the comments during a debate in the Commons on the EU Withdrawal Bill.
MPs are discussing amendments proposed by the Welsh and Scottish governments, who have criticised the bill.
But former Conservative Brexit minister David Jones rejected the move was a "power grab" during the debate.
The Clwyd West MP said the powers in devolved areas that are due to flow back from Brussels after Brexit "have never been exercised".
The bill currently proposes to transfer powers in devolved areas currently held in Brussels initially to Westminster rather than to Cardiff.
The UK government said the functions should initially be held at Westminster to allow UK-wide frameworks in areas such as farming to be developed.
Amendments are being put forward which would ensure the UK government seeks the consent of Welsh and Scottish ministers before making any changes in such policy areas.
Mr Jones said the Welsh Assembly would have "not one iota less power" and if there was to be any diminution in power it was a "theoretical one".
Fellow Tory David Davies, MP for Monmouth, did recognise it as a "power grab - grabbing powers from Brussels and bringing them back to London".
Labour's MP for Cardiff South and Penarth, Stephen Doughty, expressed "incredulity" that ministers are seeking to "undermine" the devolved administrations.
He argued the bill could enable London to "step in and make laws for Wales on devolved matters".
Earlier on Monday, Plaid Cymru called on Westminster's opposition parties to work together to change the proposed law and protect Welsh sovereignty.
In his opening remarks, Plaid's Brexit spokesman Hywel Williams said: "Devolution demands all parts of the UK have a say."
Mr Williams said: "All opposition parties must work together to protect Welsh sovereignty.
"Labour's habit of crying outrage in the media only to abstain when it comes to voting is not good enough.
"Every Labour MP must join Plaid Cymru MPs in voting against the Westminster government tonight and if every opposition MP does so, the government will be defeated."
MPs are expected to vote on certain amendments at about 01:00 GMT on Tuesday.
Last week, Wales' First Minister Carwyn Jones said he was "encouraged" to hear UK ministers conceded that changes would need to be made to the bill.
The Welsh Government supports the principle of the bill but is opposed to its potential effect on devolution.