Senedd members have voted overwhelmingly to reject the UK Government's plans for trading between the UK nations after Britain leaves the European Union.
MS' voted 36 to 15 to withhold consent to the UK Internal Market Bill. The Scottish Parliament and Northern Ireland Assembly have already refused to give their approval to it.
The UK Government described the vote as "disappointing".
Ministers in London will now have to decide whether or not to ignore the votes - they're not legally obliged to act on them, but it's considered to be an important constitutional principle that they're taken into account.
The Bill is designed to replace European Union rules and regulations that currently shape the relationships of trade and investment here in Britain.
Ministers in London have argued the new law is needed to ensure a "level playing field" for businesses in all four nations of the UK.
They say it will "protect trade and jobs across the UK by preventing new burdens on business when the Transition Period ends".
For example, a clothes manufacturer in Wales would have to follow the same subsidy and tax rules as a similar firm in Scotland or Northern Ireland.
UK cabinet ministers have said the changes will ensure a smooth transition for businesses trading mainly in the UK.
But the Welsh Government said the new rules would limit its powers to act, for example, by preventing Welsh ministers from banning single-use plastic bags.
Under the internal market rules, they wouldn't be able to prevent businesses from other parts of the UK from using single-use plastic bags.
The bill has been described as an "attack on democracy" by the Welsh Government’s Counsel General, who has said it will "steal powers" from devolved administrations.
A UK Government spokesperson said: "In conversations with businesses small and large across Wales, they have agreed that seamless trade across the UK is important for the Welsh economy to thrive.
"Around 75 per cent of Welsh exports go to England, Scotland and Northern Ireland and the UK Internal Market Bill will protect Welsh businesses and jobs by ensuring this unhindered trade continues.
"The Bill is also a hugely significant act of devolution because the end of the EU Transition period will see a huge number of powers transfer from Brussels to Cardiff Bay. We urge the UK’s devolved administrations to support this vital Bill."