Barbershop singing, as we know it today, has a direct link to the informal quartet singing in American saloons of the late 19th century. When it looked like this form of singing was about to disappear, as the safety razor made regular trips to the barbers shop unnecessary, the songs and their harmonies were collected and written down - often for the first time - so an aural folk tradition of music being passed on "by ear" was captured and developed.
The origins of unaccompanied part singing is clearly seen in the English madrigal tradition of the 16th century. Indeed Pepys and Milton were both involved in "barber music". By the late 19th century the West Gallery choirs of Southern England can be found, who used a harmonic structure very similar to that used today in barbershop music.
In the UK the first chorus to be established was the Crawley Barbershop Harmony Club, in 1965. Members of the Crawley club were instrumental in the foundation of the British Association of Barbershop Singers (BABS). This is affiliated to the American society, The Barbershop Harmony Society.
For a fuller history of barbershop there are authoritative articles by James McClelland and, in the UK, the recent publication of a detailed history by Dr Liz Calyton.