A plan by First Minister Rhodri Morgan to increase the assembly's powers is flawed, acccording to a Labour peer.
Lord Richard spent two years preparing his report
Lord Richard headed the commission which recommended law-making powers for the assembly.
But he said Mr Morgan's proposal, backed by Welsh Labour - to vary laws passed at Westminster - could not be a permanent solution.
He told BBC Wales it would also be difficult to win parliamentary approval for the plan.
His comments come after the Welsh Labour Party on Saturday endorsed a policy to give the assembly more powers.
The policy envisages two options - Mr Morgan's compromise, and the full law-making powers advocated by Welsh Secretary Peter Hain, but subject to a referendum.
Lord Richard told BBC Wales: "It would be difficult to get through Westminster.
"The House of Lords doesn't like 'Henry VIII' powers, which gives somebody the power to amend acts of Parliament by delegated legislation, statutory instruments - people don't like that."
He agreed it might work for a while but not as a "permanent runner".
"I think you can pretend for the sake of unity in the party before the general election that this is going to be the permanent solution.
"I don't think it could be permanent and you could only pretend for so long," he added.
Lord Richard believes it would be possible to win a referendum on giving the assembly full law-making powers, as his commission had made the "intellectual argument" for full devolution.
On Saturday, delegates at a Welsh Labour Party conference backed two stages of reform.
Firstly they supported the assembly being allowed to vary laws passed at Westminster.
Secondly, if Labour won the next election, they backed a consultation on a limited further extension of responsibilities, or full law-making powers.