Simon Parker argues that a Brexit would require a huge devolution of power to local government and a new federal system to mend community fault lines

One of the most frequent criticisms thrown at the Brexit campaign is that they can't explain what happens next. Would we take the Canadian, Russian or Norwegian approach to our trading relationship with our largest economic partner?

 But what neither the Brexiteers nor the Bremainers are thinking about is the domestic consequence of leaving. A vote to go could trigger a political chain reaction that will turn the already combustible debate about the British constitution into a full-on 20-megaton explosion.

The reason Brexit is so explosive is because it dramatises so precisely the real political and social cleavages in today's British society.

Modelling from the University of Bristol suggests that the people who will vote to leave are, by and large, older and less well qualified. They live in areas like the rural south and east of England, along with the fringes of old industrial cities. Those who will turn out for remain tend to be either in the devolved Celtic nations or among the younger and better-educated, living in or near the vibrant urban cores of Britain's cities.

What divides them is less about left and right and more a set of social and cultural fault lines relating to diversity and migration. If the UK Brexits, it will not take a great deal of imagination for populist politicians to fan the divisions between these two tribes into an all-out culture war.

This problem will be made worse by the political conditions that would follow a Brexit. The first thing that would happen is that the UK's already over-powerful national government would assume even more power.

It will have to make hugely divisive decisions about how to replace the EU's protection of workers' rights, our national attitude to migration and a whole host of other tricky issues. It will be dominated by a Conservative party run by jubilant Brexiteers. Without very careful handling, the fault lines will grow greater still.

For all of its many faults, the European Union has often acted to promote the rights of individuals above the rights of states.

Look at some of the issues the Brexiteers worry about the most: migration and overregulation of the labour market. With no European constraints from above and few strong institutions of local or regional government to challenge from below, our first-past-the-post system will produce an extraordinarily powerful, extraordinarily centralised executive.

The solution is that any devolution of power from Brussels to Westminster must be accompanied by massive devolution of power from Westminster to the English cities and regions.

This will allow different parts of the country to pursue and celebrate their own culture and values, and their own approaches to growing their economies. In other words, we will need a federal system for the UK.

This does not mean an English parliament, which would cover a territory so large that it would replicate all the problems of Westminster. Neither is the challenge fully answered by George Osborne's Northern Powerhouse, which currently remains a very limited form of devolution.

The exact lines on the federal map will be hotly debated and contested and some parts of the country might prefer to remain directly governed by the UK parliament.

But it is fairly easy to imagine a new Confederation of Northern Cities, stretching from Merseyside to West Yorkshire, a West Midlands grouping running from Coventry to Stoke, a new East Anglian and West of England assembly and a reformed Greater London Authority forming the building blocks of a new federal system.

In order to heal the UK's social fault lines, these new federal groupings will need a louder voice in immigration policy so they can secure a skilled workforce.

Policy should be tailored so that the cities can issue their own work permits, allowing migrants to find work within the boundaries of, say, Greater Manchester or East Anglia.

The federal assemblies will need the ability to raise and keep their own taxes, full control of NHS acute care, the criminal justice system and the job centre network, if not parts of welfare policy itself.

If national government is prepared to be sufficiently radical, the new assemblies could be paid for by the abolition of central government departments and agencies. They should have a strong voice in a reformed second chamber of the new UK federal parliament.

The debate has already started as London demands to keep more of the taxes generated in the capital, reducing the amount available to redistribute around the country. Britain's regional powerhouses may start to do the same as they begin their own economic renaissance.

If we Brexit, we will need to find new ways for different sets of values and different models of economic development to cohabit on this little island.

Simon Parker is director the New Local Government Network