Nearly 70 per cent of all babies in Barnsley are born to unmarried parents.
The town has the 10th highest number of unmarried mums and dads in the country, with 67.2 per cent of babies born out of wedlock or civil partnerships according to figures from the Office for National Statistics.
Knowsley, Blackpool and Blaenau Gwent in Wales are other areas topping the list.
Harrow in London has the lowest rate in the country, with roughly one in five babies born outside marriage.
The average for the whole of the UK is 48 per cent.
The ONS figures show that areas with the highest proportion of babies born outside marriage or civil partnerships tend to be in the north of England and Wales.
By contrast, London has seven of the 10 areas with the smallest percentage of babies born outside marriage.
Separate figures on teenage pregnancies show that Barrow-in-Furness in Cumbria has the highest rate in the UK, with roughly 17 in every 1,000 girls under 18 giving birth last year.