That is among findings by the National Audit Office in a report Progress in Setting Up Combined Authorities.

Parliament’s spending watchdog said the six – Liverpool City Region, Tees Valley, West Midlands, Greater Manchester, Cambridgeshire and Peterborough and West of England – could have been joined by other areas but these had “been unable to bring local authorities together to establish combined authorities”.

The NAO said there was “a logic to establishing strategic bodies designed to function across conurbations and sub-regional areas, and there is a clear purpose to establishing combined authorities especially in metropolitan areas, as economies and transport networks operate at a scale greater than individual local authority areas”.

Most combined authority proposals were put to the government on the basis that they would deliver more rapid economic growth by spending money and exercising powers locally.

But the NAO noted the combined authorities were “inherently complex structures” and evidence that investment, decision-making and oversight at this level was linked to improved local economic results was “mixed and inconclusive”.

It said combined authorities “often assume in their plans that there is a strong link between investment in transport and economic growth, for example”, but evidence on the additional value that governance at this level can bring to economic growth is mixed, the NAO said.

It was also concerned that combined authorities’ administrative boundaries do not necessarily match functional economic areas, or the existing boundaries of local enterprise partnerships.

“We assessed combined authorities’ draft monitoring and evaluation plans, and found that while they are working to link spending with outcomes and impact, they vary in quality, and measures tend to vary depending on data already available,” the report said.

Despite this, the combined authorities’ economic regeneration role “would become more pressing” if Brexit leads to reductions in regional funding at present received from the European Union.

Combined authorities “are generally in areas which receive the most EU funding”, it noted.

NAO head Amyas Morse said: “For combined authorities to deliver real progress and not just be another ‘curiosity of history’ like other regional structures before them, they will need to demonstrate that they can both drive economic growth and also contribute to public sector reform.”

The County Councils Network strongly opposed the government’s requirement for elected mayors to lead combined authorities – and only Cambridgeshire and Peterborough involves a county council.