Doubts have been cast as to whether a referendum on more powers for the Welsh assembly can be held this year.
In a document seen by Politics Show Wales, assembly commission chief legal advisor Keith Bush says there was not enough time for the constitutional process to be completed.
First Minister Carwyn Jones has asked Welsh Secretary Cheryl Gillan for a vote to take place this autumn.
The assembly government said it would comment after their next meeting.
In the document, Mr Best outlines the difficulties for completing the process in time for the referendum this year.
It points out that the summer recess of parliament, as well as the time needed to agree wording of the referendum, are key obstacles for an autumn referendum.
Mr Bush also sounds a note of caution over speeding up the process.
He writes: "Inadequate consultation would be a potential ground of legal challenge to the Order.
"Consultation which did not meet the requirements of the Electoral Commission would be an area of particular vulnerability," he says.
Vale of Glamorgan MP Alun Cairns accused former Welsh Secretary Peter Hain of delaying the process.
"This really blows a hole in the argument of Carwyn Jones and Ieuan Wyn Jones.
"Because of Peter Hain and the inaction of the Labour Party until the last general election it's clearly made it (an autumn referendum) nearly impossible," Mr Cairns said.
"In credit to Cheryl Gillan, she hasn't wanted to get into a political ruck with the first minister.
"We have had Ieuan Wyn Jones and Carwyn Jones saying 'we demand it now' knowing full well, because their legal advisors would have said the same thing that it's nigh on impossible.
"They have tried to put Cheryl Gillan in an impossible position but she has shown great maturity," the MP said.