AMs are to be asked to approve the use of emergency legislation laying claim to all the European Union's powers in devolved areas such as agriculture and economic development. The Welsh Government says it has to act because there's still no agreement with the UK Government about how EU powers should be distributed after Brexit.
Our political editor, Adrian Masters, says it's the nuclear option for Welsh and Scottish ministers - it would force the UK Government to refer it to the Supreme court and it could overturn it altogether but that could cause a constitutional stand off. It's also a race against time in the Assembly before the provisions of the new Wales Act come into force in April. It explicitly reserves EU matters to Westminster.
The Assembly's Presiding Officer, Elin Jones, has ruled that AMs have the right to pass the legislation under their existing powers. In Scotland, where the devolution settlement is closer to what's about to be introduced in Wales, the Presiding Officer has ruled against the legislation, although that won't stop the SNP government asking the Scottish Parliament to pass its bill.
The Scottish Government is expected to introduce similar legislation. The Welsh Conservative leader, Andrew RT Davies, has accused Carwyn Jones of "playing politics" and following the lead of the Scottish First Minister, Nicola Sturgeon.
Both Plaid Cymru and UKIP say they support the Welsh Government's position.
The UK Government has responded by saying that not only isn't Welsh legislation needed, it would be far better if the Welsh Government concentrated on agreeing amendments to the EU Withdrawal Bill, which is now in the House of Lords.