There is a risk local councillors will have limited capacity for the overview and scrutiny of combined authorities, auditors have warned.

A new report from the National Audit Office (NAO) said the creation of combined authorities was logical because economies and transport networks operate at a scale greater than individual council areas.

However, it warned this meant ‘inherently complex structures’ were being added to England’s already complicated local government arrangements.

As well as warning of the limited oversight powers councillors would have over combined authorities, the auditors said the metro mayors also campaigned on manifestos which frequently made policy commitments ‘beyond the current remits of these organisations’.

The report said the new authorities would be important post-Brexit due to their role in economic regeneration.

However, it added the capacity of most of the authorities was limited, and the lack of geographical coherence between them and other providers of public services could make devolution in the future ‘more problematic’.

‘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,’ said the head of the NAO Amyas Morse.