The Welsh Government is seeking “urgent clarification” from UK ministers that they are not preparing to take over some of its spending powers.
Wales’ European transition minister Jeremy Miles said such a move would be a “roll back” of devolution.
Proposals for a draft law due next week said the UK government will “consider which spending powers it needs” to enhance the UK internal market.
The UK government said it will confirm details of its plans “in due course”.
From next year powers which had resided with the EU will be transferred to the governments around the UK.
A draft law aimed at ensuring trade within the United Kingdom can continue smoothly under