The NAO said deals to devolve power from central government to local areas in the country are “untested” and “complex”.
However, the report said they offer opportunities to stimulate economic growth and reform public services for local users.
The government has invested £246.5m a year to enable more local control over investment decisions, and announced £2.86bn of initial allocations over five years for the first six mayoral devolution deals.
However, the NAO found significant accountability implications coming from the deals, which both central government and local areas will need to develop and clarify.
“The deals agreed so far involve increasingly complex administrative and governance configurations,” the report argued.
Details such as how and when powers will be transferred to mayors and how they will be balanced against national parliamentary accountability are yet to be clarified.
Because devolution deals are “new and experimental”, they need appropriate and proportionate measures to ensure good management and accountability, the report said.
Amyas Morse, head of the NAO, said, “Despite several iterations of deals, the government’s approach to English devolution still has an air of charting undiscovered territory.
“It is in the interests of both local areas and the government to know which programmes have the biggest impact for the money invested. Localism is not a reason for failure to learn from experiences or to spread best practice.”
The government has agreed with 10 devolution deals over the last 18 months, including in Greater Manchester, Cornwall, Sheffield City Region, the North East and the West of England.
There are 16.1 million people living in areas subject to devolution deals and nine new mayors of combined authorities will be elected in 2017.
A government spokesman said, “With 10 ground-breaking deals already agreed across the country, covering 30% of the population, next year millions of voters will be able to elect powerful new mayors directly accountable to them, particularly across the Northern Powerhouse.
“We agree there is much more to do and we will continue to talk to areas so everywhere that wants to take part in the process can do so.”
Jon Trickett, the Shadow Secretary of State for communities and local government, wrote in the New Statesman, “With no public commitment on spending from central government to help local areas implement devolution, no prediction of financial implications and no defined timetable, it’s easy to guess who will lose out.”