The assembly government wants more power over housing in Wales
A veto for the secretary of state for Wales over new housing laws for Wales may be illegal, says Welsh assembly Presiding Officer Dafydd Elis-Thomas.
Lord Elis-Thomas is seeking legal advice on the matter ahead of a House of Lords debate on the issue.
The assembly government is seeking the right to legislate on housing, including having the right to suspend the sale of council houses.
Lord Elis-Thomas says he may now vote against the UK government.
The House of Lords is due to consider the housing LCO - Legislative Competence Order - after the Easter recess.
The debate has been prompted by the tabling of a motion last week by Liberal Democrat peer Lord Livsey opposing the transfer of the powers over housing to the assembly.
Lord Livsey said he wanted the assembly to gain the powers but argued that the Welsh secretary's veto "offends an important principle of administrative law".
Lord Elis-Thomas, who is a former Plaid Cymru leader, but who sits with cross-bench peers in the Lords, warned that he believed any "constitutional doubt" could lead to problems in the future.
"The question is whether the constitution of Wales and the law-making powers of the assembly are beyond doubt," he said.
If you're not creating law which is not beyond doubt, all you're doing is creating an opportunity for future legal argument
Lord Dafydd Elis-Thomas
"The legal problems have been caused already by an attempt to produce an order which has constitutional doubt surrounding it.
"That's no proper basis for the deriving measures the new Welsh laws which we will be making and that of course effects the rights of tenants of housing associations and tenants of local authorities - this could be challenged in the courts.
"This could go on and on. If you're not creating law which is not beyond doubt, all you're doing is creating an opportunity for future legal argument."
BBC Wales political correspondent Guto Thomas said the problem hinged on points of law and of principle.
Firstly, there are serious doubts about the lack of any legal basis for such a veto in the Government of Wales Act - a concern also voiced by an expert committee of MPs and Lords last week, he said.
But such a veto could also be seen as being against the very spirit of devolution.
He said: "Voting against the government on this would not be a decision that the presiding Officer would take lightly - but given his position in Welsh politics, it would undoubtedly cause huge embarrassment for the UK Government... although this is probably part of his political calculation, in making his concerns in public now."
The Legislative Competence Order (LCO), which would transfer the powers, was approved by Whitehall departments, a committee of AMs and publicly endorsed by assembly government Deputy ousing minister Jocelyn Davies.
But MPs on the Welsh Affairs Select Committee then issued a report saying the assembly should not get the powers because they were much wider than were required.
In a statement, the Welsh Assembly Government said: "It is a priority of the Welsh Assembly Government to create more affordable housing particularly in areas of high housing pressure where there's a significant shortage in the social housing stock.
"Twenty-one months after announcing the Affordable Housing LCO it's time that we finally have these powers so that we can begin to take action in order to achieve our goals.
"Our proposals are, after all, supported by the entire housing sector in Wales and the Welsh Local Government Association.
We are therefore proceeding with the order which has been approved in the National Assembly for Wales and which we believe is legal under the Government of Wales Act 2006."
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