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Research shows that cab drivers’ grey matter enlarges and adapts to help them store a detailed mental map of the city. Those taxi drivers who were given brain scans by scientists at University College London had a larger hippocampus compared with other people. The hippocampus is a part of the brain associated with navigation in birds and other animals.

The scientists also found that part of the hippocampus grew larger as the taxi drivers spent more time in the job. “There seems to be a definite relationship between the navigating they do as taxi drivers and the brain changes,” said Dr Eleanor Maguire, who led the research team. She said: “The hippocampus has changed its structure to accommodate their huge amount of navigating experience.”

The research confirms something which London’s black-cab drivers have suspected for some time: learning their way around the capital is a highly-demanding cognitive task.

In order to drive a traditional black cab in London, drivers have to gain “the knowledge” – an intimate familiarity with the great number of streets in a six-mile radius of Charing Cross. It can take around three years of hard training, and three-quarters of those who embark on the course abandon it, according to Malcolm Linskey, manager of a London taxi school.

“There are 400 pre-established itineraries which you can be examined on, but in reality you can be asked to join any two points in the map,” he told BBC News Online. “Most people learn by visualisation but we do have a few tricks which we teach them; for example, “little apples grow quickly” gives you the order of the theatres on the north side of Shaftesbury Avenue: Lyric, Apollo, Gielgud, Queen’s.”

But “the knowledge” is definitely worth learning – drivers of black cabs are self-employed and can earn significantly more than the other type of taxi drivers (called minicabs). A black cab fare from Shepherd’s Bush to Heathrow might cost £50, compared with £28 for a minicab.