The UK cities with huge rivalry that maps show have almost merged into mega-city

Two UK cities with a longstanding rivalry could one day merge into one city.

Portsmouth and Southampton, 30 miles apart on the south coast, are slowly moving towards one another, according to a census map.

The ONS map plots residents per hectare based on 2011 census data and shows that metropolitan areas may be blending as their populations grow eastward and westward.

Portsmouth, with a total population of 208,000, has a density of 820 people per square kilometre. Southampton, with a population of 265,000, has a density of 450 people per square kilometre.

The villages and towns between the cities are getting larger too, with little countryside left and ever-increasing residential areas such as Fareham, Titchfield, and Swanwick.

It is possible that if these suburbs and the cities expand more, they will eventually merge into one mega-city - something that is slowly happening elsewhere in the country.

But this could cause local fury, with a persisting rivalry between the two cities.

Most of it is maritime-based. Southampton was the head commercial port in the district, making it more affluent. Trade coming through Portsmouth, a much smaller military port, was restricted, so there was always a civic and economic rivalry.

Historian Genevieve Bailey echoed this, adding: "After Titanic sank in 1912, sailors from Southampton refused to crew her sister ship Olympic, due to the lack of lifeboats."

She explained that dockers from Portsmouth agreed to take the place of their Southampton counterparts, which angered the Southampton workers and deepened the grudge.

The rivalry continued through the 20th century and is now most visibly seen between the two local football clubs.

The census map suggests this may also be happening between Manchester and Liverpool, showing two cities’ suburbs are slowly edging towards each other.

Back in the noughties, macro-economist Jim O’Neill proposed the idea of the northern cities combining to become a megacity.

He argued that Britain’s provincial cities are too small to compete with the high number of large urban areas boasted by the likes of China, Germany and the US.

Liverpool and Mancher aren’t the only cities that have been the subject of this sort of thinking.

Professor Roy Thompson of Edinburgh University previously said he expects the Scottish capital Edinburgh and Glasgow to amalgamate within the next three to five decades.

In the early 21st century, predictions were made that the cities’ urban areas, the centres of which are approximately 41 miles apart, could in time form a single supercity.

Professor Thompson’s work is based on a prediction concocted in the 1920s by urban planner Sir Patrick Geddes who envisioned a Central Belt called “Clydeforth” that would have the cities amalgating.