Schools will come under intense pressure as the number of four to 11-year-olds increases to its highest level since the 1970s.
Older women who have babies are increasingly choosing to do so outside marriage, an ONS report said.
It found that women of 35 and older are more likely to become mothers if they are cohabitees rather than wives.
But the disclosure of the rapidly rising numbers of broken families brought new pressure on the Coalition Government to shore up the institution of marriage.
Tuesday’s Budget contained no mention of the Tory election pledge to give tax breaks to married couples.
A further 500,000 are in cohabiting step-families, and 400,000 in married step-families.
The fig-include youngsters up to the age of 16 plus 16 to 18-year-olds in full-time education.
These figures show that the disintegration of the family is speeding up, with all the inevitable troubles that will follow.’ Jill Kirby, of the centre-right think-tank Centre for Policy Studies, said: ‘Growing up in a disrupted family at least doubles the risk for a child of doing badly at school, suffering poor health, and growing up to be unemployed, to teenage pregnancy, or to fall into crime.
‘It is crucial that the Government does something to try to give children a better start.
The equivalent of more than 2,000 extra primary schools will be needed to accommodate the extra pupils. The primary population in the capital is expected to grow by 16 per cent to 748,000 by 2014.