Over half of all children now born in Belfast are born to unmarried mothers, the report issued by the British government statistics office said.
With the divorce rate rising, marriage rates have fallen sharply in recent years as well.
Robin McRoberts of the relationship counseling agency Relate said that the reasons for the rise in illegitimacy were complex.
“A lot of it is down to changing attitudes in society,” he said. “Much of the stigma associated with living together and having children outside marriage has been forgotten.”
Rural areas as expected showed the lowest levels of illegitimate births, with Magherafelt in Derry having only 18 percent of births to unmarried mothers.
McRoberts also said that the cost of getting married and of property is forcing couples to cohabit rather than get married.
In the 1960s, there were about 100 divorces per year in the Six Counties, whereas in recent years it has reached as many as 2,300.