As English government faces its latest round of reorganisation, the minister at the head of the process has been clear about the minimum population size he expects to see when the new unitary council proposals emerge.

Simon Clarke has suggested an optimum population size of 400-500,000, with a lower limit of 300,000. His thinking is believed to have been formed by a 2006 research paper by Cardiff University on the impact of population on local authority performance.

In short, the research found three-quarters of value for money measures were impacted by the size of the authority. The report claims performance ‘tends to be better in large than small authorities' – although savings for high spend areas such as education and social services were less than those for services such as arts and leisure.

But the report has a warning: ‘The relationship between population and performance is a complex mosaic, which means that proposals for reorganisation in each local area need to be considered separately.'

Given Mr Clarke's outline figures, the Cardiff report's findings, and looking at the latest ONS population data, even the largest district councils in the country would need to be amalgamated to hit the Government's minimum size.

But it's not just the districts that are deemed not big enough. While the reorganisation debate has focused on two-tier areas, there are plenty of existing unitary councils that fail to make the grade when it comes to Mr Clarke's benchmark.

For the metropolitan authorities and even the London boroughs, there is already a devolution landscape with metro mayors. With two thirds of the London boroughs coming in at or below the 300,000 population mark, and large swathes of the northern cities made up of seemingly undersized mets, the question of a reshuffle is not yet on the cards.

Existing small unitary authorities within two-tier areas, however, could be up for grabs. Those unitaries below the supposed minimum population threshold, but at the higher end, are unlikely to face restructure for the sake of it – like Brighton and Hove City Council or the 10-year-old Central Bedfordshire, which both squeeze in just under the 300,000 mark.

But after a decade of austerity, and the fiscal squeeze created by the coronavirus pandemic, it seems some of the smaller unitaries are looking increasingly unsustainable. The list includes many of the areas economically bruised by the crisis, such as Luton, Swindon, Peterborough and Thurrock. Following the restructure of Northamptonshire as a consequence of its financial collapse, could some of the smaller unitary authorities follow in its footsteps?

With the six former Berkshire unitaries all coming in under Mr Clarke's target population – ranging from 120,000 to 220,000 – past rounds of reorganisation are looking increasingly unviable. Created in 1997, the miniscule Rutland CC has a population of under 40,000 – smaller than a district council.

As with the mets and London boroughs, small unitaries don't seem to be a big target for this latest round of reorganisation. A recent report for the County Councils' Network' – seen through the lens of potential county-based restructure – suggested county towns like Derby, Nottingham, Leicester and York could be folded back into their counties. But it conceded such a move was unlikely, and ‘these unitaries are best left as they are'.

Alternatively, they could be expanded during reorganisation – although sources close to the reorganisation debate claim the Government is reluctant to embark on any restructures that would require time consuming boundary reviews and instead it favours solutions that sit along existing county and district footprints.

When it comes to upper size limits, Simon Clarke has been more coy. A recent reference to a 600,000 maximum population limit has been given in private – which would rule out all but the smallest counties – while a public letter to North Yorkshire CC said a bid for a single unitary of 610,000 would be within the scope.

As with all reorganisations, the figures tend to gain an element of elasticity depending on who the Government is talking to. In the past, factors have included the performance of the councils and the political outcomes expected from any new authorities. This time round, economic geography seems to be the driving factor under consideration.

At 1.2m, Surrey may be double the 600,000 mark, but is putting forward a single bid. It has been indicated to the county, The MJ understands, that the Government would be willing to consider being more flexible than it would at first appear, with a unitary and devolution deal closer to that seen in Cornwall than the city regions – although with the proviso that a directly elected mayor could be introduced by the new unitary.

If Mr Clarke is willing to accept a single unitary of over a million, it seems nothing is off the cards in the latest round of reorganisation.

With the Devolution and Local Recovery White Paper expected next month, there is just a matter of weeks to wait until Mr Clarke reveals his plans.

Unitary council populations

Isles of Scilly 2,224

Rutland 39,927

Hartlepool 93,663

Darlington 106,803

Bracknell Forest 122,549

Halton 129,410

Torbay 136,264

Redcar and Cleveland 137,150

Blackpool 139,446

Middlesbrough 140,980

Isle of Wight 141,771

Slough 149,539

Blackburn with Darwen 149,696

Windsor and Maidenhead 151,422

West Berkshire 158,450

North East Lincolnshire 159,563

Reading 161,780

Wokingham 171,119

North Lincolnshire 172,292

Bedford 173,292

Thurrock 174,341

Telford and Wrekin 179,854

Southend-on-Sea 183,125

Herefordshire 192,801

Bath and North East Somerset 193,282

Stockton-on-Tees 197,348

Peterborough 202,259

Warrington 210,014

York 210,618

Luton 213,052

Portsmouth 214,905

North Somerset 215,052

Swindon 222,193

Southampton 252,520

Stoke-on-Trent 256,375

Derby 257,302

Kingston upon Hull, City of 259,778

Plymouth 262,100

Milton Keynes 269,457

Medway 278,556

South Gloucestershire 285,093

Central Bedfordshire 288,648

Brighton and Hove 290,885

County Council populations

Cumbria 500,012

Buckinghamshire 543,973

East Sussex 557,229

Somerset 562,225

Warwickshire 577,933

Worcestershire 595,786

North Yorkshire 618,054

Gloucestershire 637,070

Cambridgeshire 653,537

Oxfordshire 691,667

Leicestershire 706,155

Northamptonshire 753,278

Lincolnshire 761,224

Suffolk 761,350

Devon 802,375

Derbyshire 802,694

Nottinghamshire 828,224

West Sussex 863,980

Staffordshire 879,560

Norfolk 907,760

Hertfordshire 1,189,519

Surrey 1,196,236

Lancashire 1,219,799

Hampshire 1,382,542

Essex 1,489,189

Kent 1,581,555

*Buckinghamshire and Northamptonshire are now unitary

A Greater Essex Combined Authority is as important as unitary government