Almost three in four babies in Knowsley are born outside marriage or a civil partnership - the highest proportion in the UK.
Blaenau Gwent in Wales and Blackpool are other areas topping the list for children born to unmarried parents, according to figures from the Office for National Statistics (ONS).
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 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.
Knowsley holds the top spot with 74.8 per cent of babies, followed in second place by Blaenau Gwent (73.2 per cent).
Blackpool (72.2 per cent) and Hartlepool (70.4 per cent) are next, with Merthyr Tydfil and Redcar & Cleveland joint fifth on 70 per cent.
Hastings in East Sussex has the highest percentage in the south of England (65.6 per cent).
By contrast, London has seven of the 10 areas with the smallest percentage of babies born outside marriage.
After Harrow (20.3 per cent), the next lowest is the City of London (23.3 per cent) followed by Westminster (25.3 per cent).
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.
Council chiefs in Knowsley declined to comment.