The proportion of children born outside marriage in the UK has leapt from 12% in 1980 to 42% in 2004, according to the Office for National Statistics.
The ONS highlights a rise in unmarried, cohabiting parents
In contrast, 15 other EU countries had an estimated average of 33%, the annual ONS' Social Trends report said.
The average UK household size fell from 2.9 to 2.4 people from 1971 to 2005.
This was due to more single-parent families, smaller families and an increase in households of just one person, the ONS said.
ONS editor Hayley Butcher said: "Although most children are born to married couples, this substantial rise in births outside marriage is a reflection of the rising trend in cohabiting parents."
From 1986 to 2004, the percentage of non-married people under 60 who cohabited rose from 11% to 24% among men, and from 13% to 25% for women.
The number of households in the UK increased by 30% - from 18.6 to 24.2 million - between 1971 and 2005.
The average household size fell from 2.9 to 2.4 people over the same period.
Some seven million people lived alone in 2005 - representing 29% of all households, up from 18% in 1971 - the ONS said.
The ONS said that in the 1980s and 1990s one-person households mainly comprised older women, who tended to live longer than men.
But recent years had seen an increasing tendency for people to live alone at younger ages.
The largest increases in the last 20 years had been among people aged 25 to 44, and men from 45 to 64.
The ONS' Social Trends study also reported that young men were more likely to live with their parents than young women.
Some 57% of men aged 20 to 24 lived with their parents in 2005 - up from 50% in 1991 - compared with just 38% of women - up from 32%.
"Some young people may remain at home while in education or because of economic necessity, such as difficulties entering the housing market," the report said.
The Social Trends study's other findings included:
- In spring 2005, one in five full-time employees reported usually working more than 48 hours a week.
- Passengers travelling to or from overseas countries through UK airports (excluding transit passengers) almost quadrupled between 1980 and 2004, from 43 to 167 million.
- More than one billion journeys were made on the national rail network each year in 2003-4 and 2004-5 - the highest level since 1961.
- Transactions using debit cards increased ten-fold from 1991 to 2004, while credit card transactions trebled.
- The number of UK households that owned second homes abroad rose 45%, to almost 257,000, from 1999-2000 to 2003-4.