Welsh politicians are expected to vote against Boris Johnson’s Brexit law later.
It will not stop it from becoming law, but will mean all three of the UK’s devolved law-making bodies will have rejected the EU withdrawal agreement.
The Welsh Government said it could let UK ministers change assembly powers without Assembly Members’ consent.
Brexit Secretary Stephen Barclay said the law respected the result of the 2016 referendum.
It is expected both Labour and Plaid Cymru will reject the bill – their combined votes forming a majority in the 60-member chamber.
Under the way devolution works in the UK, the Welsh and Northern Ireland assemblies and the Scottish Parliament must give their consent to Westminster laws affecting them – including this bill.
But votes against parliamentary legislation are not legally binding.
The UK is set to leave the EU on 31 January with the deal in place.
Wales’ Brexit Minister Jeremy Miles said: “The Withdrawal Agreement Bill is careless of devolution, so as things stand, we cannot ask the Senedd to give legislative consent to the bill.
“UK ministers have not given us any real assurance that they will involve us properly in the coming negotiations with the EU – negotiations which are bound to have a major impact on Wales.
“They won’t guarantee that they will not unilaterally make changes to the devolution settlement. And they won’t give us other important protections for the interests of Wales and devolution.”
Members of the Northern Ireland Assembly voted against giving their consent for the legislation on Monday.
Its two largest parties, the Democratic Unionist Party and Sinn Fein, oppose the prime minister’s Brexit deal for different reasons. The Scottish Parliament did the same earlier in January.
The Labour Welsh Government has not backed Mr Johnson’s deal over fears it will not lead to a close enough relationship with the EU and would prove damaging to Wales’ economic interests.
Ministers, who took a pro-Remain stance prior to the December general election, have criticised the bill’s prohibition on seeking an extension to the transition period.
The government said powers for the implementation on rules on Northern Ireland “have no restrictions” and could be used to modify the assembly’s powers without its consent.
Welsh First Minister Mark Drakeford has said while the general election settled Brexit as a fact, it did not amount to “a blank cheque for the UK government to do things in a way that would do damage to the Welsh economy”.
Mr Barclay has written to the Welsh Government, asking it to change its mind: “I hope that you will revise your recommendation and support this bill,” he told Mr Miles.
“It allows us to respect the result of the referendum, in which the people of Wales voted to leave the EU, to move on to focus on other priorities and at the same time bring the country together.”
Plaid Cymru AM Delyth Jewell said her party could not support the bill because it “it threatens Welsh powers, removes parliamentary oversight of the negotiations, takes away the rights of child refugees, workers and EU citizens and unnecessarily rules out an extension to the negotiating process, making bad deal or even no deal at all the most likely outcome”.
The Brexit Party and the Conservatives will support giving the assembly’s consent to the bill.
Brexit Party’s Mark Reckless accused Welsh Labour of a “futile anti-Brexit stunt”.
“They have learned nothing from the Brexit Party’s victory in the European elections, nor the Conservative victory in December,” he said.
Welsh Conservative Senedd leader Paul Davies said the Welsh Labour Government “should respect the Welsh people’s wishes and get behind the Withdrawal Bill so we can unleash Wales’s potential”.