When media types talk about Sheffield as a hyper-creative, culturally left-field ‘Bristol of the North’ (or a Leipzig of the West), they’re talking about Sheffield A and not Sheffield B.

Sheffield A is a healthy, wealthy and leafy mix of greens, golf courses and gastropubs stretching from Fulwood and Ranmoor in the west to Nether Edge, Meersbrook and Dore in the south. This is the city that made international headlines in recent months with a campaign to protect its street trees from an incompetent and complacent council.

Sheffield B is an adjacent but almost entirely unconnected city running down the Don from Upperthorpe to Hillsborough, up to Ecclesfield in the north and stretching to Tinsley, Attercliffe, Darnall and Gleadless Valley in the east. It is a place economically characterised by poverty, lack of opportunity, low-skilled work, poor quality housing stock and even poorer public transport.

Uniquely for a British city, where pockets of deprivation are usually nestled uncomfortably between well-to-do suburbs, Sheffield’s dividing line runs directly through the city like the Berlin Wall. How did this happen?

The consensus opinion seems to be that the poorer side of the city is centred around the steel mills, factories and associated workers’ housing that doubled Sheffield’s population many times over from the Industrial Revolution right up until the decline of the steel industry in the 1980s.

The wealthier half, much of it dating from the turn-of-the-century, represents the flight of the managers and mill owners from the noise and smog of Blake’s “dark satanic mills”. The rich ensconced themselves in an enclave high above their employees, literally: Sheffield A is significantly hilly, particularly the parts that border the Peak District to the west. The spacious Victorian houses often feature spectacular views across the seven hills.

(Of course, this invisible border isn’t fixed forever. The formerly industrial area of Kelham Island has been transformed by the forces of gentrification, its proximity to the centre ensuring that its redbrick warehouses have been repurposed as gin bars, food courts and pricey flats for single professionals.)

Despite the best efforts of the pitiful privatised bus service, it is possible to cross from one city to the other. In 2013, the ‘Fairness On The 83’ project found that average life expectancy falls by 7.5 years for men and almost 10 years for women along a bus route that runs from Ecclesfield in the north to Ecclesall in the south, right across the divide. In the same year, the Sheffield Fairness Commission reported that “a baby girl born and who lives her life in one part of the city can expect to live, on average, almost 10 years longer than a similar baby girl born and living her life about four miles away, by virtue of nothing more than the socio-economic circumstances and area she was born into”. Remember, this is in one of the richest countries in the world.

We know from the work of Wilkinson and Pickett in The Spirit Level that unequal societies perform worse on almost every social metric. They’re unhealthier, unhappier and less educated, with higher rates of mental illness, property crime, obesity, infant mortality and teenage pregnancy. Their work shows that it’s not just poorer people who suffer: even the well-off do worse in societies with higher rates of economic inequality.

Sheffield has recently been labelled the ‘low pay capital’ of the UK, but you wouldn’t know that from poking around the leafy suburb of Hallam – Nick Clegg’s former constituency – which is one of the wealthiest in the entire country. The result of this division is like a real life version of China Miéville’s The City & the City. The richer half of the city don’t even see their poorer fellow citizens, as they would in London or Manchester, where disadvantaged areas like Tower Hamlets and Southwark rub shoulders with the moneyed comfort of Islington and Greenwich.

t may be cynical, but when I hear people talking up Sheffield as “the largest village in England” I sometimes wonder if what they really like about the city is that they can effectively reside in a gated community, living day-to-day without ever entering Sheffield B. Even the city centre is segregated to some degree, with the boutiques and bookshops of Division Street and the Peace Gardens forming a marked contrast to Waingate and The Wicker, two shopping streets which appear to been consigned to a barely managed decline.

The lack of interaction between these parallel cities goes some way to explaining the shock that reverberated around Sheffield A when the city as a whole voted to leave the European Union, 51 per cent leave to 49 per cent remain. For the people on the right side of the line, it hadn’t even crossed their minds that over half of their fellow citizens had been left behind. It’s a tale of two cities.