Only one baby in three in the two areas is born inside a marriage or a civil partnership.
Knowsley on Merseyside, Blaenau Gwent in Wales and Blackpool are other areas topping the list, according to figures from the Office for National Statistics.
Harrow in London has the lowest rate in the country, with roughly one in five born outside marriage.
The average for the whole of the UK is 48%.
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.
After Harrow (20.3%), the next lowest is the City of London (23.3%) followed by Westminster (25.3%).
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.
Wokingham in Berkshire and Epping Forest in Essex have the joint lowest rate of just over one in 1,000.