The system by which Welsh laws are currently made is "unclear and unaccountable", a constitutional expert has argued.
Alan Trench of University College London (UCL) appeared before MPs on the Welsh Affairs Select Committee.
He voiced criticisms of the legislative competence order or LCO process, saying it lacked clarity for voters.
LCOs are the means by which the assembly's requests for new powers are scrutinised in turn by AMs and MPs.
Once an LCO is approved at Westminster the assembly is empowered to pass its own laws in the subject area covered by the LCO, such as education and training, and vulnerable children.
These are known as assembly measures and are enforceable by the courts in the same way as acts of parliament.
Mr Trench said the LCO system, "undermines accountability and clarity for electors...it is unclear which government is responsible for what".
He was being questioned by MPs as part of their inquiry into the relationship between Wales and Whitehall.
The constitutional expert also described the 2006 Government of Wales Act, which brought the LCO system into being, as "a barrier to public understanding".
Asked about relations between ministers and civil servants in Wales and Westminster, Mr Trench claimed they had become ad hoc and messy, with variable outcomes as a result.
He told committee members he believed Whitehall had been engaged in devolution until 2002, but had paid little attention to the process since then.
Questioned over the future of the Wales Office, Mr Trench said its work had significantly increased.
He said he had, "always been puzzled that the Wales Office has been much smaller than the Scotland Office even though it has more work to do."
The Law Society and the Wales Governance Centre also gave evidence.
Dr Richard Wyn Jones of the Wales Governance Centre warned that the present settlement is leading to a very powerful executive in Cardiff Bay, which is not tightly regulated.
He claimed a failure on the part of AMs to scrutinise the assembly government adequately was leading to "executive dominance".