The logic behind this is that you need someone to take responsibility for improving the local economy, and economic areas don't coincide with local authority boundaries.

For example, employees in Birmingham firms may travel to work from outside the city. At the same time, businesses in the Black Country may be depending on Birmingham's colleges to train future staff.

It's not a new idea, and David Miliband was pushing something similar when he was Local Government Secretary in a Labour government. In those days, the term used was "city region mayors".

Will it ever happen? Perhaps in Greater Manchester or Merseyside, yes. Experience suggests getting the Black Country boroughs and Birmingham to agree to work together is a lost cause.

Having said that, Redditch ,Wyre Forest, East Staffordshire, Lichfield, Tamworth, Bromsgrove, Cannock Chase, Solihull and Birmingham are all working together in a Local Enterprise Partnership, so maybe co-operation is possible.

Here are some extracts from the report, published by The Institute for Government, of which Lord Adonis is director, and the Centre for Cities:

Our review of evidence suggests that directly elected mayors will deliver benefits because they will be able to strengthen local governance. However, the efforts of local authority mayors to support economic growth will be focused on and therefore potentially limited to the area that they represent.

As noted above, city economies often stretch far beyond the boundaries of their core local authorities. This means that a local authority mayor is likely to have a less influential role in economic development policy compared to a mayor that has authority for an extended area which relates to the wider local or regional economic area.

...

Ultimately we would like to see the introduction of metro mayors with the best aspects of the London mayoral model made available to all of England's largest cities. This would provide access to metro mayors with strategic powers over transport, planning and skills in all of England's largest cities. It will also be important to remain aware of the expansion of the London mayoral powers and consider where these should be applied to other metro mayors.

A metro mayor could take strategic decisions at a spatial scale which reflects the real economy, improving the coherence of plan-making and decision-taking. In addition, a metro mayor would be able to shoulder more powers and be a better advocate for their area both to central government as they would be elected from a larger base.

The Localism Bill should enable cities to choose to hold referenda for either metro mayors or local authority mayors in 2012. This would enable cities, where there is political appetite, to introduce a metro mayor. Where politics make this difficult a local authority mayor could still be introduced.

This flexible approach would allow cities to progress with the implementation of a mayoral system at a speed appropriate to local circumstances. The additional powers that can be afforded to a metro mayor may also make this model worth the disruption that moving to a new system of governance will inevitably cause.

More specifically, we recommend that metro mayors have the same powers to those identified for local authority mayors but over a wider area, creating the opportunity to reduce duplication and streamline policy making and local authority services.

Get the full report here: http://www.instituteforgovernment.org.uk/publications/34/